Published since 1884 by the Society for the Study of Addiction.
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What twenty years of research on cannabis use has taught us image

What twenty years of research on cannabis use has taught us

07 October 2014

In the past 20 years recreational cannabis use has grown tremendously, becoming almost as common as tobacco use among adolescents and young adults, and so has the research evidence.   A major new review in the scientific journal Addiction sets out the latest information on the effects of cannabis use on mental and physical health.

The key conclusions are:

Adverse effects of acute cannabis use

  • Cannabis does not produce fatal overdoses.
  • Driving while cannabis-intoxicated doubles the risk of a car crash; this risk increases substantially if users are also alcohol-intoxicated.
  • Cannabis use during pregnancy slightly reduces birth weight of the baby.

Adverse effects of chronic cannabis use

  • Regular cannabis users can develop a dependence syndrome, the risks of which are around 1 in 10 of all cannabis users and 1 in 6 among those who start in adolescence.
  • Regular cannabis users double their risks of experiencing psychotic symptoms and disorders, especially if they have a personal or family history of psychotic disorders, and if they start using cannabis in their mid-teens.
  • Regular adolescent cannabis users have lower educational attainment than non-using peers but we don’t know whether the link is causal.
  • Regular adolescent cannabis users are more likely to use other illicit drugs, but we don’t know whether the link is causal.
  • Regular cannabis use that begins in adolescence and continues throughout young adulthood appears to produce intellectual impairment, but the mechanism and reversibility of the impairment is unclear.
  • Regular cannabis use in adolescence approximately doubles the risk of being diagnosed with schizophrenia or reporting psychotic symptoms in adulthood.
  • Regular cannabis smokers have a higher risk of developing chronic bronchitis.
  • Cannabis smoking by middle aged adults probably increases the risk of myocardial infarction.

Professor Hall’s report is published online today in the scientific journal Addition.

-- Ends –

For editors:

Hall W. What has research over the past two decades revealed about the adverse health effects of recreational cannabis use? Addiction, 109: doi: 10.1111/add.12703

This paper is free to download for one month after publication from the Wiley Online Library or by contacting Jean O’Reilly, Editorial Manager, Addiction,, tel +44 (0)20 7848 0853.

Professor Wayne Hall has made large contribution in the field of public health in the area of drug use, addiction, treatment, ethics, and research as World Health Organization's expert adviser. As a "Highly Cited Author" identified by the Institute for Scientific Analysis, he is dedicated to public health research. He is currently working as a NHMRC Australia Fellow on addiction neuroethics (see and his research interests include alcohol and drug research and education, cancer prevention, epidemiology, health policy, mental health, pharmacoeconomics and policy, and tobacco control.

Media seeking interviews with Professor Hall may contact him by email ( or telephone (UK mobile: 0743 722 1054).

Addiction ( is a monthly international scientific journal publishing peer-reviewed research reports on alcohol, illicit drugs, tobacco, and gambling as well as editorials and other debate pieces. Owned by the Society for the Study of Addiction, it has been in continuous publication since 1884. Addiction is the number one journal in the 2013 ISI Journal Citation Reports Ranking in the Substance Abuse Category (Social Science Edition).  Membership to the Society for the Study of Addiction ( is £85 and includes an annual subscription to Addiction.