The evidence is now compelling enough to warn young people of a 40+% higher risk of developing a psychotic illness later in life if they use cannabis, according to an article published in The Lancet this week. The Lancet also has an accompanying Comment and Editorial on the same subject. The Editorial concludes that “Governments would do well to invest in sustained and effective education campaigns on the risks to health of taking cannabis.”

Approximately 20% of young people in the UK say they consume cannabis (marijuana) at least once a week. Cannabis is the most common illegal substance in the United Kingdom and the United States.

Dr Theresa Moore, University of Bristol, and Dr Stanley Zammit, Cardiff University, Wales, and team carried out a meta-analysis of 35 studies, dating up to 2006. Their aim was to investigate whether the evidence linking psychotic or mental health disorders to cannabis consumption was compelling.

The researchers discovered that people who had ever consumed cannabis were 41% more prone to develop some kind of psychotic illness, compared to people who had never consumed it. They inform that the risk intensifies according to the dose – very frequent cannabis consumers are more than twice as likely to experience some kind of psychotic illness.

The researchers also looked to see whether there was a link between cannabis consumption and suicidal thoughts and anxiety. They reported that the findings for these outcomes were less reliable.

According to recent reports, say the authors, about 40% of young adults and adolescents have ever used cannabis. This means that if the 41% higher risk of a psychotic illnesses is linked to those who have ever used cannabis, approximately 14% of psychotic outcomes in British young adults would not have occurred if cannabis had never been consumed.

“We have described a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders. Despite the inevitable uncertainty, policymakers need to provide the public with advice about this widely used drug. We believe that there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life,” the authors conclude.

“In the public debate, cannabis has been considered a more or less harmless drug compared with alcohol, central stimulants, and opioids. However, the potential long-term hazardous effects of cannabis with regard to psychosis seem to have been overlooked, and there is a need to warn the public of these dangers, as well as to establish a treatment to help young frequent cannabis users.” Drs Merete Nordentoft and Carsten Hjorthoj, Copenhagen University Hospital, Denmark, say in the accompanying Comment.

Written by: Christian Nordqvist