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Would decriminalising personal use of cannabis lead to higher rates of mental illness?

BMJ 2020; 368 doi: (Published 15 January 2020) Cite this as: BMJ 2020;368:l6975
  1. Bobby P Smyth, clinical senior lecturer, Department of Public Health and Primary Care1,
  2. Mary Cannon, professor, Department of Psychiatry2,
  3. Andrew Molodynski, honorary senior clinical lecturer, Department of Psychiatry3,
  4. H Valerie Curran, director, Clinical Psychopharmacology Unit and professor of psychopharmacology4,
  5. Niamh Eastwood, executive director, Release5,
  6. Adam R Winstock, founder and chief executive officer, Global Drug Survey, honorary clinical professor, Department of Epidemiology, and consultant addiction psychiatrist6
  1. 1Trinity College Dublin, Ireland
  2. 2Royal College of Surgeons in Ireland, Dublin, Ireland
  3. 3Oxford University, UK
  4. 4University College London, UK
  5. 5Release, London, UK
  6. 6University College London, UK
  1. Correspondence to: B P Smyth smythbo{at}, H V Curran v.curran{at}

Removing criminal penalties for possession could increase adolescent use, say Bobby P Smyth, Mary Cannon, and Andrew Molodynski. But H Valerie Curran, Niamh Eastwood, and Adam R Winstock find no evidence for this and say that liberalisation of drug laws could reduce harms

Yes—Bobby Smyth, Mary Cannon, Andrew Molodynski

Here we consider two main questions. Firstly, does cannabis use lead to higher rates of mental illness? Secondly, would decriminalising cannabis lead to increased use? The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) describes decriminalisation as a policy response to drug use where use remains prohibited but criminal penalties for possession of small quantities for personal use are removed and replaced by an alternative civil sanction. The supply of cannabis would remain a crime.1

Does cannabis use lead to higher rates of mental illness?

The answer to the first question is a resounding yes. But don’t just take our word for it. The US National Academies of Sciences, Engineering, and Medicine carried out the most comprehensive and rigorous study of recent research on the health effects of cannabis.2 Their conclusion was that cannabis use was likely to increase the risk of developing schizophrenia, other psychoses, social anxiety disorder, depression, and suicidal thoughts. This was backed up by a further systematic review this year.3

It’s especially worrying that these risks are most pronounced in young people.2 Cannabis use among young people is also linked to subsequent lower educational attainment and higher rates of unemployment and low income.2 These adverse social outcomes constitute predisposing and perpetuating risk factors for mental ill health.

The weight of scientific evidence linking cannabis use to increased rates of mental illness is very strong. Cannabis dependence contributes more to morbidity among adolescents worldwide than any other illicit drug.4 Therefore, it’s clear that any policy change that increases cannabis use, especially by …

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