Public Release:  What 20 years of research on cannabis use has taught us

Wayne Hall, WHO Expert Advisor on addiction, reviews cannabis research since 1993

Wiley

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.

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

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