What have we learned from two decades of cannabis research? Well, according to World Health Organization addiction adviser Professor Wayne Hall, it all breaks down to the following, as published in the journal Addiction (full text available here for one month):
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 heart attack.
As you can see, nothing here comes out of the blue -- and it's all from the standpoint of addiction research rather than medicinal use, etc. The warning against adolescent cannabis use stands out strongly in the chronic use section.