Cannabis used for non-medical purposes commonly exists as either a resin (called hashish in the United States) or as dry herbal material. The resin is made by compressing resin glands from the plant. The dried plant material is dried flowers and fruits and some leaves and stems of the female cannabis plant.  The plant can be processed into a number of products including oils, concentrates and extracts.

Cannabis contains more than 120 chemicals known as cannabinoids. Experts still aren’t sure what each cannabinoid does, but two are relatively well-studied – CBD (cannabidiol) and THC (Δ9-tetrahydrocannabinol), with THC being the most studied. CBD is a non-intoxicating cannabinoid and THC is the main psychoactive compound that is responsible for the ‘high’ feeling.[1] In the plant, THC and CBD exist in their acid form (THC-A, or CBD-A), which get converted to THC or CBD on heating or drying. The combined effect of CBD and THC is complex and not well understood, but it is known that the ratio is important because CBD reduces some of the more negative aspects of THC, including anxiety and unpleasant psychosis.[2] Levels of THC and CBD vary depending on the plant strain and growing conditions. The strength of cannabis has been steadily increasing on the illegal market through the breeding of plants to increase the THC content, lower the CBD content, or make the ratio of THC to CBD higher.[3]

When cannabis is smoked, THC (and other chemicals) pass from the lungs to the blood, which carries them rapidly throughout the body to the brain. Effects are felt quickly. When cannabis is eaten, the effects are delayed by 30-60 minutes as the body digests the food or drink. This delay may cause some people to eat more THC, leading to a higher dose than planned. The noticeable effects of cannabis last about 1-3 hours if smoked and can last for many hours when eaten. Cannabinoids can stay stored in the body for periods lasting weeks or more, which has implications for drug testing.

See ‘Is all cannabis use harmful? and ‘How might legalisation change the health impacts of cannabis?’

[1] Devinsky et al., “Cannabidiol: Pharmacology and Potential Therapeutic Role in Epilepsy and Other Neuropsychiatric Disorders,” Epilepsia 55, no. 6 (2014); Banister et al., “Dark Classics in Chemical Neuroscience: Delta(9)-Tetrahydrocannabinol,” Acs Chemical Neuroscience 10, no. 5 (2019); Di Marzo, “New Approaches and Challenges to Targeting the Endocannabinoid System (Vol 17, Pg 623, 2018),” Nature Reviews Drug Discovery 17, no. 9 (2018)

[2] Boggs et al., “Clinical and Preclinical Evidence for Functional Interactions of Cannabidiol and Delta(9)-Tetrahydrocannabinol,” Neuropsychopharmacology 43, no. 1 (2018); Di Forti et al., “High-Potency Cannabis and the Risk of Psychosis,” British Journal of Psychiatry 195, no. 6 (2009)

[3] Chandra et al., “New Trends in Cannabis Potency in USA and Europe During the Last Decade (2008-2017) (Vol 269, Pg 5, 2019),” European Archives of Psychiatry and Clinical Neuroscience 269, no. 8 (2019)

It is very common to have tried cannabis and common to occasionally use cannabis even though it is illegal. Most people have tried cannabis at some point in their lives and 15% of people reported using cannabis in the past year in 2018/19.[1] It is not clear how rates of cannabis use would change if recreational use is legalised but it is reasonable to expect that legalisation will ‘normalise’ cannabis use in society. Exact changes depend on whether non-users begin to use cannabis upon legalisation, and whether existing users change their patterns of use. In turn, regulation of the legal market and the price of legal cannabis will impact use patterns. The regulation will dictate where, how much and what types of cannabis can be accessed legally.

It is very common to have tried cannabis and common to occasionally use cannabis even though it is illegal

Through studies and surveys conducted locally, we know that despite cannabis being illegal:

• Most New Zealanders have tried cannabis. Large numbers of people report trying cannabis,[2] with one study finding four out of five people tried cannabis before the age of 25.[3] We tend to see higher rates of use reported where study participants are followed over time as opposed to one-off questionnaires which may reflect differences in peoples’ willingness to report honestly or differences in the types of people captured by the study.
• Overall, an increasing number of New Zealanders report current use. The number of people who reported using cannabis at least once in the past year increased from 8% in 2011/12 to 15% in 2018/19.[4] Māori report rates of use that are roughly double those of non-Māori.[5] There is limited information about the rates of cannabis use for Pacific people, though available data suggests that young people who live in the Pacific Islands have higher rates of use than young Pacific people who live in Aotearoa New Zealand.[6]
• Many people who use cannabis do so frequently. Around a third of New Zealanders who use cannabis report using it at least once every week.[7]
• Younger New Zealanders are the biggest users of cannabis. Around 29% of people aged 15-24 years report using cannabis in the last year, up from 15% in 2011/12.[8] Though rates of use are going up in 15-24 year olds, fewer young New Zealanders are reporting that they have tried cannabis in year 10 with continued decreases in use between 2001 and 2018. Five percent fewer year 10 students reported ever trying cannabis in 2018 compared to 2012 and 2% fewer reported using cannabis in the past month. The largest drop in use has been in Māori, younger students and students at lower decile schools, and the changes are not thought to be because students were increasing use of other drugs.[9]
• Getting a cannabis conviction doesn’t stop people from continuing to use cannabis. Ninety-five percent of people who use cannabis either continue or increase their cannabis use after arrest or conviction.[10]
• Using both cannabis and tobacco is common. People who smoke tobacco are about five times more likely to also use cannabis, and about 30 times more likely to smoke both if they are heavy users.[11] In a global drug survey, around one in four New Zealanders who use cannabis reported mixing with tobacco, well below the global average of three in five.[12]

There are only a small number of studies to date measuring the impacts on rates of use where cannabis has been legalised. These short-term effects may not reflect longer-term trends. This is because the legal retail environment is only recently established in most places, so the impact of legalisation is lagging behind the law change. Some studies are also limited in how they measure ‘use’, commonly looking at whether someone used cannabis in the past month or year which may not reflect problematic use that leads to increased harm. Use trends differ by age, gender and location so there are no blanket findings about cannabis use following legalisation.

Use trends differ by age, gender and location so there are no blanket findings about cannabis use following legalisation

So far, the evidence from overseas shows that in the short term:

• Adults report moderate increases in occasional cannabis use: The Canadian government reports that after legalisation around 2% more people used cannabis in the past three months, reflecting particular increases in use by males, adults over 25 and those from certain regions.[13] US studies also report increases in past-year use ranging from around 1-5%,[14] and highlight changes in certain subgroups, including an increase in past-month use in college students[15] and people older than 25.[16] In Uruguay, a 1% increase in past-year and 1.6% increase in past-month cannabis use have been reported between 2011 and 2014.[17] Notably rates of occasional cannabis use were increasing before legalisation so these increases may reflect ongoing trends.
• Patterns of frequent and problematic use may change: In the US, the frequency of cannabis use has increased among adult cannabis users after legalisation of medical cannabis with commercial retail outlets and similar evidence is emerging after legalisation for recreational use.[18] There is early evidence of a small increase in cannabis use disorder from 0.9% to 1.23% for those over 26 in states where recreational legalisation has occurred compared to those where it hasn’t.[19] However, in Canada there hasn’t been an increase in the number of people using cannabis daily or almost daily, except in those over 65.[20]
• There is no clear pattern of change for young people. In the US, there has been no clear impact on rates of use or cannabis use disorders in youth. Studies have demonstrated increases and decreases of around 2-4%, or no change, depending on who is surveyed and in which states. One study found that the increase in rates of cannabis use disorder in 12-17 year olds is 25% higher in states where cannabis has been legalised, but these findings may be caused by other factors, and no differences were reported for those aged 18-25.[21] In Canada, there has been no reported change in those age 15-24 for occasional or frequent use. A significant drop in occasional use is reported for 15-17 year olds, but it is stated this needs to be interpreted with caution.[22] In Uruguay, a study found no impact of cannabis legalisation on rates of youth use.[23] It is possible these trends reflect wider decreases in substance use seen among young people.[24]

There are only a small number of studies to date measuring the impacts on rates of use where cannabis has been legalised. These short-term effects may not reflect longer-term trends

It is too early to tell what the impact of legalisation on use rates will be and we are unlikely to know long-term outcomes on patterns of use for some time. For context, it took four decades for alcohol consumption to return to pre-prohibition levels in the US.[25]

We also need to interpret cannabis use data with caution. Because it is illegal, research on cannabis has historically been difficult and there are real concerns about collecting and storing data on who uses cannabis and where they get it from. People may also be reluctant to be honest about their use of cannabis when it is illegal but more willing to report use when it becomes legal. This makes it look like use has increased following legalisation, though it doesn’t explain differential changes among different subgroups. Alternative methods of testing cannabis use such as through wastewater testing, as was done in Washington State,[26] could get around these issues. In Aotearoa New Zealand, ESR is working with the police on testing for drugs in wastewater, but cannabis is not currently included.

Public health impacts are related to problematic use of cannabis rather than occasional use, so health impacts depend on who is using cannabis and how often – see ‘Is all cannabis use harmful?

[1] Ministry of Health, “New Zealand Health Survey: Annual Data Explorer”, 2019

[2] Ministry of Health, “Cannabis Use 2012/13: New Zealand Health Survey”, 2015

[3] Boden et al., “Illicit Drug Use and Dependence in a New Zealand Birth Cohort,” Australian & New Zealand Journal of Psychiatry 40, no. 2 (2006)

[4] Ministry of Health, “New Zealand Health Survey: Annual Data Explorer”, 2019

[5] Ministry of Health, “Cannabis Use 2012/13: New Zealand Health Survey”, 2015

[6] Howard et al., “Alcohol, Cannabis and Amphetamine‐Type Stimulants Use among Young Pacific Islanders,” Drug and alcohol review 30, no. 1 (2011)

[7] Ministry of Health, “Cannabis Use 2012/13: New Zealand Health Survey”, 2015; Fergusson et al., “Psychosocial Sequelae of Cannabis Use and Implications for Policy: Findings from the Christchurch Health and Development Study,” Social Psychiatry and Psychiatric Epidemiology 50, no. 9 (2015)

[8] Ministry of Health, “Cannabis Use 2012/13: New Zealand Health Survey”, 2015

[9] Ball et al., “Declining Adolescent Cannabis Use Occurred across All Demographic Groups and Was Accompanied by Declining Use of Other Psychoactive Drugs, New Zealand, 2001–2012,” New Zealand Medical Journal 132, no. 1500 (2019); Jude Ball, “Adolescent Cannabis Use Continues Its Downward Trend, New Zealand 2012–2018,” New Zealand Medical Journal 133, no. 1510 (2020)

[10] Lenton, “Cannabis Policy and the Burden of Proof: Is It Now Beyond Reasonable Doubt That Cannabis Prohibition Is Not Working?,” Drug and Alcohol Review 19, no. 1 (2000)

[11] Badiani et al., “Tobacco Smoking and Cannabis Use in a Longitudinal Birth Cohort: Evidence of Reciprocal Causal Relationships,” Drug and Alcohol Dependence 150 (2015)

[12] Winstock et al., “Global Drug Survey 2019-Key Findings Report”, 2019

[13] Rotermann, “What Has Changed since Cannabis Was Legalized?” Health reports 31, no. 2 (2020)

[14] Kerr et al., “Changes in Marijuana Use across the 2012 Washington State Recreational Legalization: Is Retrospective Assessment of Use before Legalization More Accurate?,” Journal of Studies on Alcohol and Drugs 79, no. 3 (2018); Bahji et al., “International Perspectives on the Implications of Cannabis Legalization: A Systematic Review & Thematic Analysis,” International Journal of Environmental Research and Public Health 16, no. 17 (2019)

[15] Kerr et al., “Oregon Recreational Marijuana Legalization: Changes in Undergraduates’ Marijuana Use Rates from 2008 to 2016,” Psychology of Addictive Behaviors 32, no. 6 (2018)

[16] Cerdá et al., “Association between Recreational Marijuana Legalization in the United States and Changes in Marijuana Use and Cannabis Use Disorder from 2008 to 2016,” JAMA Psychiatry 77, no. 2 (2020)

[17] de Drogas, “Vi Encuesta Nacional En Hogares Sobre Consumo De Drogas, 2016,” Informe de investigación (2018)

[18] Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019); Cerdá et al., “Association between Recreational Marijuana Legalization in the United States and Changes in Marijuana Use and Cannabis Use Disorder from 2008 to 2016,” JAMA Psychiatry 77, no. 2 (2020)

[19] Cerdá et al., “Association between Recreational Marijuana Legalization in the United States and Changes in Marijuana Use and Cannabis Use Disorder from 2008 to 2016,” JAMA Psychiatry 77, no. 2 (2020)

[20] Rotermann, “What Has Changed since Cannabis Was Legalized?” Health reports 31, no. 2 (2020)

[21] Cerdá et al., “Association between Recreational Marijuana Legalization in the United States and Changes in Marijuana Use and Cannabis Use Disorder from 2008 to 2016,” JAMA Psychiatry 77, no. 2 (2020)

[22] Rotermann, “What Has Changed since Cannabis Was Legalized?” Health reports 31, no. 2 (2020); Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019); Smart et al., “Early Evidence of the Impact of Cannabis Legalization on Cannabis Use, Cannabis Use Disorder, and the Use of Other Substances: Findings from State Policy Evaluations,” The American Journal of Drug and Alcohol Abuse 45, no. 6 (2019); Cerdá et al., “Association of State Recreational Marijuana Laws with Adolescent Marijuana Use,” JAMA Pediatrics 171, no. 2 (2017); Dilley et al., “Prevalence of Cannabis Use in Youths after Legalization in Washington State,” JAMA Pediatrics 173, no. 2 (2019); Melchior et al., “Does Liberalisation of Cannabis Policy Influence Levels of Use in Adolescents and Young Adults? A Systematic Review and Meta-Analysis,” BMJ Open 9, no. 7 (2019); Cerdá et al., “Association between Recreational Marijuana Legalization in the United States and Changes in Marijuana Use and Cannabis Use Disorder from 2008 to 2016,” JAMA Psychiatry 77, no. 2 (2020); Mennis et al., “Adolescent Treatment Admissions for Marijuana Following Recreational Legalization in Colorado and Washington,” Drug and Alcohol Dependence 210 (2020)

[23] Laqueur et al., “The Impact of Cannabis Legalization in Uruguay on Adolescent Cannabis Use,” International Journal of Drug Policy 80 (2020)

[24] Looze et al., “Decreases in Adolescent Weekly Alcohol Use in Europe and North America: Evidence from 28 Countries from 2002 to 2010,” The European Journal of Public Health 25, no. suppl_2 (2015)

[25] Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019)

[26] Burgard et al., “Using Wastewater‐Based Analysis to Monitor the Effects of Legalized Retail Sales on Cannabis Consumption in Washington State, USA,” Addiction 114, no. 9 (2019)

Doctors use the term ‘cannabis use disorder’ to describe problematic use of cannabis, ranging from mild to severe, with addiction at the severe end of the spectrum. Cannabis use disorder can lead to a person suffering from anxiety, sleeping problems, depression and appetite changes when they stop using it. People who use cannabis have a 1 in 5 risk of developing cannabis use disorder, with risk increasing for those who use cannabis early, often, and use more potent cannabis.[1] But use alone does not determine if someone will develop problematic use of cannabis – other personal, environmental and social factors will contribute.

[1] Lichtman et al., Cannabinoid Tolerance and Dependence (Springer, 2005); Leung et al. “What is the prevalence and risk of cannabis use disorders among people who use cannabis? A systematic review and meta-analysis.” Addictive Behaviors (2020)

Illegal cannabis use is common and the majority of people who use cannabis have not experienced harms from their use. But the consequences of using cannabis aren’t the same for everyone.[1] Certain people or those with particular patterns of use are more likely to be harmed – either because of health impacts from using cannabis (see below) or because cannabis is illegal and using it has got them in trouble with the law (referred to as ‘social’ harm – see ‘What are the social and community impacts of cannabis prohibition?’). Cannabis also comes in a variety of forms and strengths – some are less harmful than others – see ‘What is cannabis?’

The consequences of using cannabis aren’t the same for everyone

• Cannabis use can be harmful for young people. The earlier in life you begin using cannabis, the higher your risk of serious health problems. As a general rule, using cannabis before age 15 leads to the poorest outcomes and waiting until age 25 to use cannabis has the best outcomes. People who start using cannabis at a young age are less likely to finish their education and more likely to be unemployed, use other drugs, experience mental health conditions and develop cannabis use disorder (a diagnosis given for problematic cannabis use, including addiction and other related problems), especially if they use cannabis often.[2] See ‘What is cannabis use disorder?’
• Using cannabis daily or nearly every day is more harmful than occasional use. People who use cannabis frequently or intensively are more likely to experience poor outcomes or develop problems with use, especially young people.[3] People who use cannabis often also pay more for it, most likely because the price fluctuates seasonally but they cannot wait for lower prices.[4]
• Higher potency cannabis is associated with some mental health problems. Stronger cannabis has higher concentrations of THC and/or a higher THC to CBD ratio, which is known to increase the risk of developing mental health conditions or cannabis use disorder.[5] Most cannabis within Aotearoa New Zealand is thought to be high THC with low levels of CBD.[6]
• All ways of using cannabis come with risks. Different ways of using cannabis, including smoking, vaping and edibles have different risks.[7]
  • Guidelines for lower-risk use of cannabis advise people to avoid smoking it. Similar to smoking cigarettes, smoking cannabis has negative health impacts such as lung damage, including so-called ‘bong lung’.[8] The Government has set a goal for smokefree 2025, aiming for fewer than 5% of New Zealanders to be smokers by that date. This is not a law change and smoking will not be illegal – it’s simply a target to support people who smoke to quit and prevent non-smokers from starting smoking.
  • Vaping is currently considered a lower-risk alternative to smoking cannabis. Although vaping cannabis is likely to be substantially less harmful than smoking cannabis, it is not harmless. A big concern is that we don’t know what the long-term health effects of vaping are because it is so new. Differences in vape pens (e.g. different maximum temperatures) and products (e.g. dry flower vs vaping liquid) are associated with different health-related harms. For example, use of cannabis vaping liquids has been associated with a cluster of severe lung injury cases caused by contamination of the products with vitamin EMost of these were illicitly produced and without quality controls contamination by bacteria or other chemicals could also happen. Another concern with vaping is that it enables people to use higher-potency cannabis concentrates, increasing the risk of harm.
  • Consuming cannabis through edibles is considered lower-risk than smoking it. Cannabis edibles are anything that you can eat or drink that has been infused with cannabis. Using cannabis this way eliminates the substantial respiratory risks and lung damage associated with smoking cannabis. It can also reduce risk if edible products are available with regulated potency. Edibles have other potential issues that need to be managed.[9] Unfamiliarity with edible dosing and difficulties in dividing edibles can result in unintentional overdose.[10] This may be particularly problematic for first time or novice users. Accidental exposure to edibles is a particular risk for children and is a risk whether or not the product is legal. However, following legalisation of recreational cannabis in US states, there has been an increase in the number of children who accidentally ate products containing cannabis, leading to intoxication and sometimes hospitalisation.[11]
• Cannabis use is higher risk for people with a personal or family history of mental health conditions. People who are vulnerable to mental health conditions, including psychosis, are more likely to develop a condition after using cannabis.[12] It’s safest for people with a personal or family history of these conditions to avoid using cannabis altogether.
• Using cannabis during pregnancy has risks of causing harm to the mother and baby. Maternal cannabis use increases the risk of anaemia, low birthweight, and childhood developmental and behavioural problems.[13] It’s safest for pregnant women to avoid using cannabis altogether.
• Cannabis use is higher risk for people with heart conditions. Cannabis use increases a person’s heart rate in a dose-dependent way and might make heart conditions worse.[14] It’s safest for people with a history of heart disease to avoid using cannabis altogether.

Most health-related harm is suffered by those who use cannabis young, at high potency and on most days – this harm happens whether or not cannabis is legal. Lower-risk cannabis use guidelines have been developed by researchers based on evidence of the least harmful approaches to cannabis use.[15] Questions have been raised about the likelihood of people following these guidelines as there is currently little or no support for addiction treatment and education to steer people towards lower-risk use of cannabis.[16] The strategies to reduce harm include:

1. Choosing not to use cannabis
2. Not starting to use cannabis until after 25
3. Using cannabis products with a low THC content or higher CBD:THC ratio
4. Not using synthetic cannabis products
5. Using non-smoking options to consume cannabis
6. If smoking, avoiding deep inhalation and long holding of the breath
7. Minimising use as much as possible
8. Not driving or operating machinery while impaired
9. Completely avoiding cannabis use if you have a personal or family history of psychosis, substance use problems, or are pregnant
10. Avoid combining behaviours that are considered higher-risk

 See ‘How might legalisation change the health impacts of cannabis?’

[1] Fergusson et al., “Psychosocial Sequelae of Cannabis Use and Implications for Policy: Findings from the Christchurch Health and Development Study,” Social Psychiatry and Psychiatric Epidemiology 50, no. 9 (2015); Silins et al., “Young Adult Sequelae of Adolescent Cannabis Use: An Integrative Analysis,” Lancet Psychiatry 1, no. 4 (2014); Anthony, “The Epidemiology of Cannabis Dependence,” Cannabis dependence: Its nature, consequences and treatment  (2006); van der Pol et al., “Predicting the Transition from Frequent Cannabis Use to Cannabis Dependence: A Three-Year Prospective Study,” Drug and Alcohol Dependence 133, no. 2 (2013); Budney et al., “The Cannabis Withdrawal Syndrome,” Curr Opin Psychiatry 19, no. 3 (2006); Poulton et al., “Patterns of Recreational Cannabis Use in Aotearoa-New Zealand and Their Consequences: Evidence to Inform Voters in the 2020 Referendum,” Journal of the Royal Society of New Zealand (2020)

[2] Fergusson et al., “Psychosocial Sequelae of Cannabis Use and Implications for Policy: Findings from the Christchurch Health and Development Study,” Social Psychiatry and Psychiatric Epidemiology 50, no. 9 (2015); Silins et al., “Young Adult Sequelae of Adolescent Cannabis Use: An Integrative Analysis,” Lancet Psychiatry 1, no. 4 (2014); Anthony, “The Epidemiology of Cannabis Dependence,” Cannabis dependence: Its nature, consequences and treatment  (2006); van der Pol et al., “Predicting the Transition from Frequent Cannabis Use to Cannabis Dependence: A Three-Year Prospective Study,” Drug and Alcohol Dependence 133, no. 2 (2013); Budney et al., “The Cannabis Withdrawal Syndrome,” Curr Opin Psychiatry 19, no. 3 (2006); Poulton et al., “Patterns of Recreational Cannabis Use in Aotearoa-New Zealand and Their Consequences: Evidence to Inform Voters in the 2020 Referendum,” Journal of the Royal Society of New Zealand (2020)

[3] Patton et al., “Cannabis Use and Mental Health in Young People: Cohort Study,” British Medical Journal 325, no. 7374 (2002); Poulton et al., “Patterns of Recreational Cannabis Use in Aotearoa-New Zealand and Their Consequences: Evidence to Inform Voters in the 2020 Referendum,” Journal of the Royal Society of New Zealand (2020)

[4] Wilkins et al., “Determinants of the Retail Price of Illegal Drugs in New Zealand,” International Journal of Drug Policy 79 (2020)

[5] Hall et al., “High Potency Cannabis”, British Medical Journal 350 (2015)

[6] Sheehan et al., “Chemical and Physical Variations of Cannabis Smoke from a Variety of Cannabis Samples in New Zealand,” Forensic Sciences Research 4, no. 2 (2019)

[7] Fischer et al., “Lower-Risk Cannabis Use Guidelines: A Comprehensive Update of Evidence and Recommendations,” American Journal of Public Health 107, no. 8 (2017)

[8] Gracie et al., “Cannabis Use Disorder and the Lungs,” Addiction (2020)

[9] Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019); Fischer et al., “Lower-Risk Cannabis Use Guidelines: A Comprehensive Update of Evidence and Recommendations,” American Journal of Public Health 107, no. 8 (2017)

[10] Zipursky et al., “Edible Cannabis,” Canadian Medical Association Journal 192, no. 7 (2020)

[11] Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019)

[12] Degenhardt et al., “Cannabis and Psychosis,” Current Psychiatry Reports 4, no. 3 (2002); Caspi et al., “Moderation of the Effect of Adolescent-Onset Cannabis Use on Adult Psychosis by a Functional Polymorphism in the Catechol-O-Methyltransferase Gene: Longitudinal Evidence of a Gene X Environment Interaction,” Biological Psychiatry 57, no. 10 (2005); Poulton et al., “Patterns of Recreational Cannabis Use in Aotearoa-New Zealand and Their Consequences: Evidence to Inform Voters in the 2020 Referendum,” Journal of the Royal Society of New Zealand (2020)

[13] Metz et al., “Marijuana Use in Pregnancy and Lactation: A Review of the Evidence,” American Journal of Obstetrics and Gynecology 213, no. 6 (2015); Gunn et al., “Prenatal Exposure to Cannabis and Maternal and Child Health Outcomes: A Systematic Review and Meta-Analysis,” BMJ Open 6, no. 4 (2016)

[14] Volkow et al., “Adverse Health Effects of Marijuana Use,” New England Journal of Medicine 370, no. 23 (2014)

[15] Fischer et al., “Lower-Risk Cannabis Use Guidelines: A Comprehensive Update of Evidence and Recommendations,” American Journal of Public Health 107, no. 8 (2017)

[16] Caulkins et al., “Lower-Risk Cannabis Use Guidelines: Will Users Listen?,” American Journal of Public Health 110, no. 1 (2019)

The health impacts of cannabis use aren’t the same for everyone. Some can be beneficial and some harmful.[1] It depends on who is using it, what they are using it for and how often. The health impacts also depend on the type of cannabis being used. Cannabis is variable – it comes in different strengths and products, different parts of the plant can be consumed, and sometimes it is combined with other substances. Cannabis use has health impacts regardless of whether it is legal or illegal, but the impacts may change depending on how the legal market is regulated.

Cannabis use has health impacts regardless of whether it is legal or illegal, but the impacts may change depending on how the legal market is regulated

• Trying cannabis is unlikely to cause harm, but ongoing use can have negative health impacts. As well as the intoxicating effects experienced immediately after use, heavy cannabis use can have a range of health impacts. Research into the health impacts of cannabis has been limited by its illegal status and it is difficult to assess the adverse health impacts that are specifically caused by cannabis. What we do know has been summarised by the Royal Society Te Apārangi in Cannabis: How it affects our health. In short:
  • Cannabis use can be addictive but most people don’t become addicted.
    • See ‘What is cannabis use disorder?’
  • Cannabis use can alter brain development. This can occur for people who start using cannabis regularly or intensively early in adolescence while the brain is still developing, but can change back after time without use.[2] Regular use over a long time can also lead to reduced memory and attention span.[3]
  • Smoking cannabis can cause lung damage. People who smoke cannabis may experience symptoms of chronic bronchitis and other respiratory symptoms such as cough and wheeze.[4]
  • Cannabis use can increase the risk of psychotic symptoms or psychosis in some people. Cannabis can contribute to psychotic symptoms – when people experience changes in how they perceive reality and find it hard to know what is real – but most people that use cannabis do not experience these symptoms and most cases are not caused by cannabis.[5] Psychotic symptoms associated with cannabis use are often very mild.[6] Cannabis can also contribute to more serious psychotic illness and the risk of this outcome is substantially increased for people who start using cannabis young, often and in more potent forms.[7] Around one in ten people who started using cannabis before they were 15 will go on to develop psychosis by age 26, and it is more likely for those who used it often.[8] People with psychosis who keep using cannabis have a worse prognosis than those who stop using it.[9] The risk is higher for people who have a family history of psychotic illness. Part of the link might also be explained by people suffering from psychosis being drawn to using cannabis.[10]
  • There is a weak association between cannabis use and depression. This could suggest a causal link or could be explained by people with depression being more likely to use cannabis.[11] We cannot draw a definitive conclusion about the direction of effect.
• Changes in rates of use may impact the prevalence of these health conditions.
  • See ‘Will cannabis use increase if it is legalised?’
• Potency limits may reduce some negative health impacts. The more potent the cannabis, the more likely it is to cause health-related harms. In the illicit market, cannabis potency has been steadily increasing.[12] For example in the illicit market in the USA, the average THC level from seized cannabis increased from around 4% in 1996 to around 17% in 2017, with an increase in THC:CBD ratio from 23 to 104.[13] The proposed regulation includes a limit on THC of 15% for dried plant material, which is thought to be the upper end of the illegal market in Aotearoa New Zealand, though there is limited data on this.[14] We would expect a range of low THC and high CBD to also be available. The THC limit needs to strike a balance between reducing harm and drawing users away from the illegal market. A potential consequence of a lower limit is that it could maintain an illegal market for higher potency cannabis. Limits on other products are yet to be determined, but provisions for concentrates also indicate higher-strength cannabis would be available. Stronger cannabis could still be grown through home grow provisions in the law.
• Age restrictions may limit young people using cannabis, but could lead to them accessing illicit cannabis instead. Preventing people under 20 from using cannabis would reduce the negative health impacts associated with adolescent use. Though the number of young teenagers trying cannabis has been declining in recent years, young people up to age 25 are still the highest users of cannabis.
  • See ‘Is all cannabis use harmful?’
• Public health messaging and education may help to promote lower-risk use. A benefit of legalising cannabis is that it becomes easier to educate the public about the risks and harms associated with cannabis use, including public health messaging about who might be more likely to be harmed from use. Because of the increased harm from adolescent use, specific preventative programmes aimed at school-age children may reduce harm.
  • See ‘Is all cannabis use harmful?’
• If legal, there may be better access to health services that can support people with cannabis addiction.
  • See ‘Will legalising cannabis burden the healthcare system?’

[1] Royal Society Te Apārangi, “Cannabis: How It Affects Our Health,” (2019); Russell et al., “Routes of Administration for Cannabis Use – Basic Prevalence and Related Health Outcomes: A Scoping Review and Synthesis,” International Journal of Drug Policy 52 (2018); Fischer et al., “Lower-Risk Cannabis Use Guidelines: A Comprehensive Update of Evidence and Recommendations,” American Journal of Public Health 107, no. 8 (2017); Solowij et al., “Cognitive Functioning of Long-Term Heavy Cannabis Users Seeking Treatment,” JAMA 287, no. 9 (2002); Solowij et al., “Cannabis and Cognition: Short and Long-Term Effects,” Marijuana and Madness 2 (2012); Solowij et al., “Verbal Learning and Memory in Adolescent Cannabis Users, Alcohol Users and Non-Users,” Psychopharmacology 216, no. 1 (2011)

[2] Lorenzetti et al., “Adolescent Cannabis Use, Cognition, Brain Health and Educational Outcomes: A Review of the Evidence”. European Neuropsychopharmacology (2020); Kroon et al., “Heavy Cannabis Use, Dependence and the Brain: A Clinical Perspective,” Addiction 115, no. 3 (2020)

[3] Solowij et al., “Cognitive Functioning of Long-Term Heavy Cannabis Users Seeking Treatment,” JAMA 287, no. 9 (2002)

[4] Poulton et al., “Patterns of Recreational Cannabis Use in Aotearoa-New Zealand and Their Consequences: Evidence to Inform Voters in the 2020 Referendum,” Journal of the Royal Society of New Zealand (2020); Gracie et al., “Cannabis Use Disorder and the Lungs,” Addiction (2020)

[5] Volkow et al., “Adverse Health Effects of Marijuana Use,” New England Journal of Medicine 370, no. 23 (2014); Hall et al., “Adverse Health Effects of Non-Medical Cannabis Use,” The Lancet 374, no. 9698 (2009); Karila et al., “Acute and Long-Term Effects of Cannabis Use: A Review,” Current Pharmaceutical Design 20, no. 25 (2014); Degenhardt et al., “The Global Epidemiology and Contribution of Cannabis Use and Dependence to the Global Burden of Disease: Results from the Gbd 2010 Study,” PLOS ONE 8, no. 10 (2013); Hall, “What Has Research over the Past Two Decades Revealed About the Adverse Health Effects of Recreational Cannabis Use?,” Addiction 110, no. 1 (2015); Volkow et al., “Effects of Cannabis Use on Human Behavior, Including Cognition, Motivation, and Psychosis: A Review,” JAMA Psychiatry 73, no. 3 (2016); Broyd et al., “Acute and Chronic Effects of Cannabinoids on Human Cognition—a Systematic Review,” Biological Psychiatry 79, no. 7 (2016); Scott et al., “Association of Cannabis with Cognitive Functioning in Adolescents and Young Adults: A Systematic Review and Meta-Analysis,” JAMA Psychiatry 75, no. 6 (2018); Fergusson et al., “Cannabis and Psychosis,” BMJ 332, no. 7534 (2006); Poulton et al., “Patterns of Recreational Cannabis Use in Aotearoa-New Zealand and Their Consequences: Evidence to Inform Voters in the 2020 Referendum,” Journal of the Royal Society of New Zealand (2020)

[6] Pearson et al., “Cannabis and Psychosis through the Lens of Dsm-5,” International Journal of Environmental Research and Public Health 16, no. 21 (2019)

[7] Murray et al., “Will Legalization and Commercialization of Cannabis Use Increase the Incidence and Prevalence of Psychosis?,” JAMA Psychiatry  (2020)

[8] Arseneault et al., “Cannabis Use in Adolescence and Risk for Adult Psychosis: Longitudinal Prospective Study,” BMJ 325, no. 7374 (2002); McLaren et al., “Assessing Evidence for a Causal Link between Cannabis and Psychosis: A Review of Cohort Studies,” International Journal of Drug Policy 21, no. 1 (2010)

[9] Murray et al., “Will Legalization and Commercialization of Cannabis Use Increase the Incidence and Prevalence of Psychosis?,” JAMA Psychiatry (2020)

[10] Fergusson et al., “Cannabis and Psychosis,” BMJ 332, no. 7534 (2006)

[11] Fergusson et al., “Tests of Causal Linkages between Cannabis Use and Psychotic Symptoms,” Addiction 100, no. 3 (2005); Rey et al., “Mental Health of Teenagers Who Use Cannabis: Results of an Australian Survey,” The British Journal of Psychiatry 180, no. 3 (2002); Arseneault et al., “Cannabis Use in Adolescence and Risk for Adult Psychosis: Longitudinal Prospective Study,” BMJ 325, no. 7374 (2002)

[12] Wilkins et al., “Cannabis and Other Illicit Drug Trends in New Zealand, 2005,” Auckland, Massey University (2005)

[13] Chandra et al., “New Trends in Cannabis Potency in USA and Europe During the Last Decade (2008-2017) (Vol 269, Pg 5, 2019),” European Archives of Psychiatry and Clinical Neuroscience 269, no. 8 (Dec 2019)

[14] Sheehan et al., “Chemical and Physical Variations of Cannabis Smoke from a Variety of Cannabis Samples in New Zealand,” Forensic Sciences Research 4, no. 2 (2019)

A key goal of legalising cannabis is to shift cannabis use towards ‘health issue’ and away from ‘criminal issue’. This can be achieved by reducing harm caused by problematic use of cannabis and lowering the use of cannabis over time through education and addiction services. Because of this, legalising cannabis could change demands on the healthcare system for addiction services, emergency needs and longer-term health needs.

Legalising cannabis could change demands on the healthcare system for addiction services, emergency needs and longer-term health needs

• People who need help to manage their use of cannabis aren’t seeking it and there’s not enough help available. The illegal status of cannabis can deter people from seeking treatment, for fear of condemnation, judgement or arrest. There is currently no or very minimal resource for treatment of cannabis-related harms, especially for young people, and availability of treatment for people with cannabis use disorder varies widely across the country. Currently, only a small proportion of people who have cannabis problems seek help to manage their use. In 2012/13, around one in 100 New Zealanders who use cannabis had received help to reduce their level of use in the last year and around one in 25 people who wanted help did not get it.[1] For the roughly 10% of people who use cannabis and suffer from unwanted psychotic symptoms, there are very limited resources for recovery and treatment to address problematic use of cannabis and psychosis. Unlike tobacco, alcohol and opioids, there are no proven medications to treat cannabis use disorder and very limited specialist treatments for are available. Behavioural interventions and school-based interventions are known to be modestly effective and could be made more widely available within a legal framework.[2]
• Māori suffer disproportionately from addiction issues. Inequitable health services and access to treatment, among other factors, have contributed to Māori being significantly impacted by cannabis use disorder.[3] The social harms of addiction on Māori communities include whānau breakdowns, economic deprivation, criminal offending and reoffending, housing difficulties, employment issues and stigmatisation.[4]
  • See ‘How would legalisation of cannabis impact Māori specifically?’
• If legal, more people may seek help and more help could be available. People are likely to be more confident to seek help for problematic use of cannabis if it is legal because of a decrease in stigma or fear of prosecution. In the 2014 Global Drug Survey, New Zealanders answered that they strongly agreed that drug policy liberalisation would increase confidence in seeking help.[5] The law proposes to reduce harm through education and health and social services so better support is likely to be available for people who have problematic use of cannabis through revenue raised by cannabis sales. Legalisation has been too recent overseas to know how demands on addiction treatment services may change.[6]
  • See ‘How much revenue would the Government make if cannabis was legal?’
• Increased demands for addiction treatment might happen anyway. People may be more open to asking for help for a legalised drug, but even in countries where cannabis remains illegal (throughout Europe and Australia) more people are seeking treatment to help quit or control their addiction to cannabis.[7] The recent changes to the Misuse of Drugs Act to take a more health-oriented approach mean that regardless of whether the law changes, demands on treatment services for people with cannabis problems may increase. More help needs to be available, though this would be without a potential funding source from cannabis sales.
• Other cannabis-related health impacts could increase demands on the healthcare system. Legalisation in overseas jurisdictions has been too recent to know how demands on treatment services change for other health issues such as bronchitis or chronic psychosis disorders.[8] Early evidence from the US showed that emergency department visits for cannabis-related presentations increased following the legalisation of recreational cannabis, with several studies reporting the very low base rate roughly doubled.[9] Early evidence also suggests an increase of around 5 more poison exposures being reported per million people each quarter in US states after legal cannabis was available through licensed dispensaries. No significant burden on the healthcare system has been reported elsewhere, suggesting that any changes are small.
  • See ‘How might legalisation change the health impacts of cannabis?’

[1] Ministry of Health, “Cannabis Use 2012/13: New Zealand Health Survey”, 2015

[2] Vogl et al., “A Universal Harm-Minimisation Approach to Preventing Psychostimulant and Cannabis Use in Adolescents: A Cluster Randomised Controlled Trial,” Substance Abuse Treatment, Prevention, and Policy 9, no. 1 (2014)

[3] Walker, “Issues of Tobacco, Alcohol and Other Substance Abuse for Māori”, 2019; Theodore et al., “Cannabis, the Cannabis Referendum and Māori Youth: A Review from a Lifecourse Perspective,” Kōtuitui: New Zealand Journal of Social Sciences Online (2020)

[4] Walker, “Issues of Tobacco, Alcohol and Other Substance Abuse for Māori”, 2019

[5] Benfer et al., “The Impact of Drug Policy Liberalisation on Willingness to Seek Help for Problem Drug Use: A Comparison of 20 Countries,” International Journal of Drug Policy 56 (2018)

[6] Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019)

[7] Hall et al., Cannabis Use and Dependence: Public Health and Public Policy (Cambridge University Press, 2003); European Monitoring Centre for Drugs and Drug Addiction (EMCDDA), “Table Tdi-105, Part Vii: All Clients Entering Inpatient Treatment by Primary Drug and Age, 2009 or Most Recent Year Available: All Cannabis Inpatient Clients by Country and Age.”, 2011; World Health Organization, Atlas on Substance Use (2010): Resources for the Prevention and Treatment of Substance Use Disorders (Geneva, 2010); Roxburgh et al., “The Epidemiology of Cannabis Use and Cannabis‐Related Harm in Australia 1993–2007,” Addiction 105, no. 6 (2010)

[8] Leung et al., “What Have Been the Public Health Impacts of Cannabis Legalisation in the USA? A Review of Evidence on Adverse and Beneficial Effects,” Current Addiction Reports 6, no. 4 (2019)

[9] Reed, Impacts of Marijuana Legalization in Colorado: A Report Pursuant to Senate Bill 13-283 (Colorado Department of Public Safety, Division of Criminal Justice, 2018); Kim et al., “Colorado Cannabis Legalization and Its Effect on Emergency Care,” Annals of Emergency Medicine 68, no. 1 (2016); Wang et al., “Marijuana and Acute Health Care Contacts in Colorado,” Prev Med 104 (2017)

Estimates of the potential revenue from a legal cannabis market in Aotearoa New Zealand need to be interpreted with caution because many estimates were made prior to the details of the parameters for taxes, levies and licensing fees being shared in the final draft bill. All estimates also depend on the size of the market, which is uncertain. Rather than being a profit-making enterprise for the government, it is more helpful to consider that a legal framework will allow for more meaningful spending associated with cannabis use – shifting from enforcement costs to health-related costs.

• The government cannot source revenue from cannabis sales while it is illegal. Under prohibition, it is impossible to generate tax revenue or benefit from economic development from cannabis production or sales.
• A legal market can generate revenue for the government through taxes. The amount of revenue from a legal cannabis market depends on total sales and the way the government taxes sales. If tax is a proportion of the sale, tax revenue would decrease as the price of cannabis decreases unless this is offset by an increase in total sales. The taxation level and price of legal products will be critical factors in the extent to which consumers shift from illegal to legal retail sources.[1]
• Spending on enforcement currently outweighs spending on harm reduction. Under prohibition, the government spends substantially more on cannabis enforcement than cannabis-focused prevention or professional help (e.g. treatment) services through the health system. It is possible these spending patterns could change under prohibition if enforcement lessened.
• There will be new and different costs associated with a legal market. The development of infrastructure and administration to regulate cannabis and improved health services will require additional expenditures. Enforcement of remaining offences also carries a cost.
• Health resources for cannabis users could be supported more meaningfully under a legal framework. Legalisation provides an opportunity to save money on expenditures for enforcement of prohibition and generate tax revenue from cannabis production or sales. This revenue could be directed to health-oriented and harm-reduction measures related to cannabis.

[1] Hammond et al., “Evaluating the Impacts of Cannabis Legalization: The International Cannabis Policy Study,” International Journal of Drug Policy 77 (2020)