Intended for healthcare professionals

Head To Head Head to Head

Should the supply of cannabis be legalised now?

BMJ 2019; 366 doi: (Published 03 July 2019) Cite this as: BMJ 2019;366:l4473
  1. Molly Meacher, cross bench peer and co-chair of the All-Party Parliamentary Group for Drug Policy Reform1,
  2. David Nutt, director, Neuropsychopharmacology Unit2,
  3. Jonathan Liebling, director, Cannabis Patient Advocacy and Support Services3,
  4. Robin M Murray, professor of psychiatric research4,
  5. Adam Gridley, mental health writer and recovered cannabis user5
  1. 1House of Lords, London
  2. 2Imperial College London
  3. 3Reading
  4. 4Institute of Psychiatry, King’s College, London
  5. 5London
  1. Correspondence to: Molly Meacher meachermc{at}, R M Murray robin.murray{at}

Strict regulation would allow for healthier cannabis consumption than the criminally controlled current supply can offer, say Molly Meacher, David Nutt, and Jonathan Liebling. But Robin Murray and Adam Gridley worry that legalisation could increase cannabis use and associated psychiatric disorders

Yes—Molly Meacher, David Nutt, Jonathan Liebling

Opponents of legalisation and regulation of the supply of cannabis raise legitimate fears that it could lead to more use and therefore increased mental health harms. However, recent research in the US, where some states have legalised cannabis supply for adult social use, suggests that cannabis consumption has increased irrespective of its legal status in each state.1

Legalising and regulating cannabis confers many benefits over the current illegal and unregulated supply. Here we focus on benefits that directly affect mental and physical health.

Making consumption safer

The priority for drug policy must be to protect the mental and physical health of teenagers by discouraging cannabis use among this group and, in particular, consumption of cannabis with high concentrations of tetrahydrocannabinol (THC)—“skunk”—or contaminated products. We can expect that a proportion of teenagers who currently use skunk will access the regulated product, just as they access alcohol at present. A strong educational campaign will be important to warn of the risks associated with use. In US states with legal, regulated cannabis sales the number of teenagers taking cannabis has not risen, and in Colorado it has fallen.2

States that legalise cannabis supply can provide a safer environment for young people, but this will depend on the regulatory environment. For example, smoking cannabis with tobacco should be discouraged, and vaping of cannabis should be encouraged to reduce lung damage.

A key health issue is the composition of cannabis products. The drug may contain as many as 140 different cannabinoids in varying proportions. The two main ones are THC—often called the active ingredient—and CBD (cannabidiol), a non-intoxicating, possibly antipsychotic cannabinoid.3

A recent study has shown just how important the composition of cannabis products is in reducing health risks to the consumer. Cannabis supply remains illegal for social use in the UK, but cannabis is nevertheless used by an estimated 2.1 million people in England and Wales aged 16-59.4 They buy it from drug dealers operating illegally, who want to maximise profits without concern for their customers’ health. Cannabis products with high THC concentrations yield the biggest profits and are therefore the dealers’ favoured offering.

Skunk is associated with psychotic episodes, particularly in young people with a family history of psychosis. It may also inhibit teenagers’ brain development.

The crucial question is whether low potency cannabis can be consumed safely. Di Forti and colleagues concluded that “individuals who mostly used low-potency (hash-like) cannabis occasionally, at weekends or daily had no increased likelihood of psychotic disorders compared with those who never used cannabis.”5 On the other hand, people who used skunk daily were more than five times as likely to have a psychotic disorder diagnosed.5

Benefits of a regulated market

Legalisation would enable the state to regulate the industry, with legal outlets supplying only herbal cannabis with balanced THC and CBD. Regulations would include an age threshold of 18 or 20 for cannabis purchase, and educational campaigns would have to clarify that, although such cannabis is much safer for adults than alcohol or tobacco, it may carry risks for young people whose brains are still developing.

A regulated market could have other health benefits when compared with the current situation. Regulation would ensure labelling: users would know the strength of the cannabis they were buying and could be sure that the product was free from adulterants, pesticides, and other harmful substances. Alcohol consumption would be likely to decline.6

Currently, the only access to cannabis is through dealers operating illegally (apart from the few UK patients who have a private prescription for medical cannabis). Dealers also offer crack cocaine and heroin, risking a gateway effect between illegal cannabis use and the purchase of class A drugs. To maximise profits dealers promote these more dangerous and addictive drugs to children despite their health risks,7 so legalisation of cannabis supply could well reduce the use of other, more harmful illegal drugs. We await research evidence that this logical benefit of cannabis legalisation has occurred in those states that have introduced reforms.

Finally, cannabis sale could be taxed and the income invested in the NHS, in treatment for drug disorders, and in drug education in schools. Rogeberg has shown that a state controlled regulated market gives the best overall outcomes.8 State controlled regulated markets avoid the tendency among markets run for profit to incentivise increased use. Tighter controls than those for tobacco and alcohol regulation would be needed: for example, we would recommend a maximum THC limit and a minimum CBD limit in all legal products. Advertising would be banned.

In conclusion, legalisation and regulation of cannabis supply for adult social use would create a safer environment for children and young people and would reduce the health risks associated with cannabis.

No—Robin M Murray, Adam Gridley

A huge pharmacological experiment is currently under way with the brains of young Americans. It would be wise to watch how the results unfold before deciding whether to follow their example.

The UK’s legalisation of medicinal cannabis means that cannabis based products can be prescribed to some people who may benefit from them. Furthermore, recreational cannabis use has in practice been decriminalised in most areas of the UK.9 But what would happen if recreational cannabis markets were legalised and the supply and sale of cannabis were legally regulated? This would take part of the trade out of the hands of some criminals—but not all, as no one is proposing to make cannabis available to 15 year olds, and experience in North America is that the illicit market continues to flourish.

Legalisation holds the promise of tax revenues, and it will certainly make investors rich. A recent report projected annual legal sales of cannabis at $66.3bn (£52.4bn; €58.4bn) by 2025.10 Tobacco and alcohol companies are jumping on board. Altria, the owner of Marlboro cigarettes, bought a $1.8bn share in a Canadian cannabis company in 2018,11 while Anheuser-Busch, the world’s largest beer company, is investing $50m in researching drinks infused with cannabis.12

Increased risk of psychosis

Cannabis is reasonably safe in terms of physical health: smoking it can damage the lungs, but this is mostly a result of the tobacco it’s commonly mixed with. However, about one in 10 users becomes dependent, and numerous prospective studies have shown that cannabis use carries an increased risk of later, schizophrenia-like psychosis. In the heaviest users this risk increases by five13 to nine times,14 so 10% of heavy users of high potency cannabis are likely to develop psychosis.14 Cannabis use also increases the risk of depression and suicide, although the effect is not as strong as that for psychosis.15

The adverse effects of tobacco and alcohol track the extent of their use. Lung cancer reached epidemic proportions after cigarette smoking spread, and liver disease increases in proportion to alcohol consumption. Similarly, with cannabis, Di Forti and colleagues showed that the incidence of psychosis throughout 11 areas in Europe correlated highly (r=0.8) with the prevalence of daily cannabis use in the general population in the different sites. Indeed, if no one smoked high potency cannabis, 30% of cases of psychosis would be prevented in London, and 50% would be prevented in Amsterdam, where Nederhash—containing up to 60% tetrahydrocannabinol (THC) —can be lawfully smoked in “coffee” shops.14

Throughout the US, cannabis use and dependence have both increased among adults in the states that have legalised cannabis for medicinal use.16 These states also show higher rates of cannabis use disorders17 and serious mental illness.18 It’s too early to see the mental health effects of legalising recreational cannabis. However, Colorado, the first state to legalislate for recreational use, has five times as many “pot shops” as it has McDonald’s outlets, its use is 85% higher than in the rest of the US,19 and potency has sky rocketed.

In the UK we consider skunk to be high potency, but on average it contains only 14% THC. In Colorado, “shatter” and “wax dabs” concentrates are widely available, containing as much as 90% THC. It’s clear that legalisation has driven up potency.

Cannabis related visits to the University of Colorado Hospital’s emergency department trebled in the five years after legalisation from under 250 to over 800,20 and the share of drivers involved in fatal crashes who tested positive for cannabis increased from 11% in 2013 to 21% (n=139) in 2017.21

Heavy use

It’s often said that alcohol and tobacco are more harmful to society than cannabis, but their heavy use is far more common because they are legal. Is it inevitable that legalisation in the UK would result in more dependence and psychosis? In theory it should be possible to legalise without an increase in use and potency, but the precedents from the US aren’t encouraging. In our opinion, the best option at present is to adopt the Portuguese model, where cannabis use is decriminalised but supply is not legalised, and the police refer persistent users to treatment.


  • Molly Meacher and Robin Murray were due to debate this question at the Royal College of Psychiatrists’ international congress in London on 2 July 2019.

  • Competing interests (Meacher et al): We have read and understood BMJ policy on declaration of interests and declare the following interests: MM is co-chair of the All Party Parliamentary Group for Drug Policy Reform. In this capacity MM has received grants from the Open Society Foundation for Drug Policy research and projects. OSF is a not-for-profit organisation.

  • DN sits on the advisory boards of several research and pharmaceutical companies, which have no connection to medical cannabis. DN has been paid to speak by several pharmaceutical companies that do not produce medical cannabis products. DN founded and chairs the charity DrugScience, which is funded by individual donations, a grant from Open Society Foundations, book sales, and companies that produce cannabis. DN’s company Alcarelle seeks to develop less harmful recreational alternatives to alcohol.

  • JL is employed as lead researcher by the not-for-profit Centre for Medicinal Cannabis and is cofounder and director of the not-for-profit Cannabis Patient Advocacy and Support Services. Both organisations exclusively focus on medicinal access for patients. Neither organisation is concerned with campaigning or lobbying for the legal regulation of cannabis for adult use.

  • Competing interests (Murray et al): We have read and understood BMJ policy on declaration of interests and have the following interests to declare: RM has received honorariums for giving lectures for Janssen, Otsuka, Lundbeck, and Sunovian. He has attended one meeting of Canopy Growth Health regarding CBD. He is a member of the Cannabis-Based Medicinal Products Panel, advising the Advisory Committee for Drug Misuse and the Home Office.

  • Provenance and peer review: Commissioned; not externally peer reviewed.


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