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

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.