20 years of cannabis research: what have we learned?

Over the past 20 years, the use of cannabis has become almost as common as tobacco among adolescents and young adults. A new review by Professor Wayne Hall from the National Addiction Centre at King's College London summarises the scientific evidence gathered between 1993 and 2013 on the effects of cannabis use on mental and physical health. 

Professor Hall, who is also Director of the Center for Youth Substance Abuse Research at the University of Queensland, says,  ''Over the past 20 years, we have seen a large increase in the number of people smoking cannabis. As would be expected, we've also seen a large increase in the number of research studies on cannabis. What's clear is that cannabis, especially when users smoke it regularly and from a young age, can have a detrimental impact on people's mental health. 

''We are seeing larger numbers of people becoming dependent on cannabis, and it is now difficult to argue that cannabis dependence does not require professional attention. Whilst the mental and physical impact of cannabis dependence is less severe than alcohol or heroin dependence, the number of people who are able to stop smoking cannabis completely following treatment is still very low.''

The key conclusions from the report, published in the journal Addiction, are: 

• Driving while intoxicated with cannabis doubles the risk of road traffic accidents. In comparison, being intoxicated with alcohol increased the risk of a crash 6-15 times.
• Approximately 9 per cent of people who have ever used cannabis become dependent, compared to 32 per cent for nicotine, 23 per cent for heroin, and 15 per cent for alcohol. 
• Maternal cannabis use during pregnancy modestly reduces birth weight. 
• Daily cannabis users double their risk of experiencing psychotic symptoms and disorders, especially if they have a personal or family history of psychosis, and if they start using cannabis in their mid-teens
• Daily cannabis use that begins in adolescence and continues through adulthood appears linked to cognitive impairment, but the mechanism and whether this is reversible remains unclear. 
• People who smoke cannabis daily as teenagers are more likely to use other illicit drugs, but some evidence suggests the relationship may be due to shared risk factors. 
• Smoking cannabis increases the risk of cardiovascular disease, mainly because most cannabis users have smoked, or still smoke tobacco as well.