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Published 12 December 2019

Health impacts of cannabis – benefits, harms and many unknowns

A new report by Royal Society Te Apārangi summarises what is known about the potential risks and benefits to health from the use of recreational and medicinal cannabis.

It finds evidence that cannabis-based medicines have some therapeutic effect in specific clinical situations and that recreational cannabis use can have some negative health outcomes. The report finds large knowledge gaps in understanding the health effects of both recreational cannabis and cannabis-based medicines, particularly in specific groups of the population, and this lack of knowledge poses a public health risk.



Royal Society Te Apārangi President Professor Wendy Larner says the report draws heavily on the 2017 publication by the US National Academies of Sciences, Engineering and Medicine: The Health Effects of Cannabis and Cannabinoids: The Current State of Evidence and Recommendations as well as additional literature of relevance to Aotearoa.

“With changes to New Zealand’s legislation on prescribing cannabis and the upcoming referendum on whether New Zealand should legalise recreational cannabis, Royal Society Te Apārangi has prepared this report to assist people considering the opportunities and risks to health associated with cannabis use.”

Associate Professor Joe Boden, Department of Psychological Medicine, University of Otago, who was one of the report’s reviewers, says that for medicinal cannabis, there is evidence of effectiveness to treat a small number of specific conditions but the evidence is not well established as it hasn’t been widely tested in clinical trials.  “Cannabis-based medicines have been shown to have some therapeutic effects in treating chronic pain, nausea from chemotherapy, muscle spasms due to multiple sclerosis and in rare forms of epilepsy, but we don’t have good evidence that they work better than the existing medicines available to treat these conditions.”

Dr Irene Braithwaite, Medical Research Institute of New Zealand, who also was a reviewer of the report, says that cannabis-based medicines, many of which are derived from plant material, have not been through the same vigorous efficacy or safety testing as existing medicines. “The levels and types of cannabinoids in these plant-based preparations can vary greatly due to different growing, harvest and storage conditions. This makes it difficult to draw conclusions by comparing or pooling the results from the limited number of studies that do exist,” says Dr Braithwaite. 

For recreational use, some harms are also seen. “Recreational cannabis is associated with negative outcomes including mental illness, particularly in youth, drug use disorders, respiratory illness, impaired cognition, increased road accidents and lower birthweight in babies born to women exposed to cannabis,” says Professor Michelle Glass, Head of the Department of Pharmacology and Toxicology at University of Otago. “People may assume cannabis is largely safe as it has been used by people for so long, but this is not necessarily the case.”

The report finds many gaps in the research literature, particularly in population groups thought to be at most risk from cannabis use: those under 18 years, pregnant and breastfeeding women, people over 50 years and heavy cannabis users. 

An area where the risks are clear is synthetic cannabis, says Professor Glass, who studies cannabinoids in her research. 

“Calling it synthetic cannabis is misleading as it’s not cannabis. It’s non-cannabis plant material sprayed with lab-made chemicals that mimic the effect of cannabis in the body but tend to be much more powerful. In 2017 there were approximately 45 deaths linked to synthetic cannabinoids in New Zealand and every day, ambulance services in New Zealand report multiple life-threatening cases related to synthetic cannabinoid use. 

“Being so different to cannabis, it can be difficult to identify with standard drug screening techniques.”

Professor Glass says it is worth noting that synthetic cannabis would remain illegal, regardless of the outcome of the public referendum, such is its great potential to cause harm. 

View report Cannabis: How it affects our health 


Key facts about cannabis and cannabinoids

(see full report for more detail and references)


Q: What is cannabis?

A: Cannabis grows as separate male and female plants and produces a large number of bioactive compounds. The best known are THC (Tetrahydrocannabinol) that produces a ‘high’ when taken and CBD (cannabidiol) that doesn’t but has other effects on the body.


Q. Why does cannabis affect us?

A. Some compounds in cannabis, like THC, affect us because they interact with our own cannabinoid system. We have cannabinoid receptors in our brain and gut, in particular, and we produce cannabinoids to regulate appetite, form memories, calm fear, affect our movement and posture and regulate our immune system.


Q: How long have humans used cannabis as a medicine?

A: Written records show cannabis was used for medicinal purposes in China as far back as 6,000 years ago.


Q. What medical conditions can cannabis treat?

A. The evidence for cannabis-based medicine is not well established as it hasn’t been widely tested in clinical trials. There is evidence that cannabis-based medicines may have some effect as an additional treatment option for controlling chronic pain in adults, reducing nausea and vomiting from chemotherapy, easing pain from muscle spasms in people with multiple sclerosis and treating two rare forms of severe epilepsy in children.


Q. How do people take medicinal cannabis?

A. Medicinal cannabis is usually taken as a spray, oil, capsule or as pharmaceutical grade plant material.


Q. What cannabis-based medicines are available in New Zealand?

A. Medical cannabis preparations with low levels of the psychoactive compound, THC, can be prescribed for palliative care and the oral spray Sativex® can be prescribed for those with multiple sclerosis. Changes are underway, however, to allow doctors to prescribe medicinal cannabis more widely.


Q. What risks are associated with the way people take cannabis?

A. The safest form is pharmaceutical products such as sprays, oils or capsules. Smoking cannabis exposes users to toxic compounds created at high temperatures and it can lead to a chronic cough and phlegm production. Vaporising at lower temperatures (such as vaping) also exposes a user to toxic compounds from additives, solvents and flavouring. Edible forms can lead to excessively high doses and accidental ingestion. Synthetic cannabis poses the greatest risk because the active chemicals are much stronger than natural cannabis and can have longer lasting and more dangerous effects on the body.


Q. What is synthetic cannabis?

A. Synthetic cannabis is non-cannabis plant material sprayed with chemicals that mimic the effect of cannabis but often has a stronger effect on the body so is more risky to take. Approximately 45 deaths in New Zealand were linked to synthetic cannabis use in 2017 and ambulance services report multiple life-threatening cases related to synthetic cannabis use every day. Its use would remain illegal regardless of the outcome of the recreational cannabis referendum.


Q. How does recreational cannabis affect physical health?

A. Smoking cannabis regularly can cause a chronic cough and phlegm production. It can affect brain development and short term memory, particularly in young people, making learning more difficult. It can also lead to drug use disorders. Maternal cannabis exposure can lead to lower birthweights and negatively affect a baby’s brain and nerve development. Cannabis use can also lead to increased road accidents.


Q. How does recreational cannabis affect mental health?

A. Cannabis may have an adverse effect on mental health but more research is needed to confirm this. It may increase the risks of schizophrenia and psychosis, bipolar disorder, depression and anxiety. Also, people can develop problem cannabis use and may become addicted.


Q. Is it safe to drive after using cannabis?

A. Driving under the influence of cannabis increases the risk of having a crash. The amount of cannabis at which driving becomes unsafe is unknown and impairment from cannabis is difficult to measure.


Q. What more do we need to know about the effects of cannabis?

A. The lack of quality information on the effects of both medicinal and recreational cannabis poses a public health risk because we don’t know enough about how safe or effective it is. Urgent research is needed on population groups thought to be at most risk: those under 18, pregnant and breastfeeding women, people over 50 years and heavy cannabis users.


Source: Royal Society Te Apārangi