There is now enough evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life by more than 40%, conclude authors of an Article published in this week's edition of The Lancet. The issues are also explored in an accompanying Comment and Editorial, with the Editorial concluding: "Governments would do well to invest in sustained and effective education campaigns on the risks to health of taking cannabis."
Cannabis, or marijuana, is the most commonly used illegal substance in most countries, including the UK and USA. Up to 20% of young people now report use at least once per week or heavy use (use on more than 100 occasions).
Dr Theresa Moore, University of Bristol, and Dr Stanley Zammit, Cardiff University, Wales, and colleagues did a meta-analysis of 35 studies, dated up to 2006, to assess whether there was evidence to connect cannabis use to occurrence of psychotic or mental health disorders.
They found that individuals had used cannabis ever were 41% more likely than those who had never used the drug to have any psychosis. The risk increased relative to dose, with the most frequent cannabis users more than twice as likely to have a psychotic outcome. Depression, suicidal thoughts, and anxiety outcomes were examined separately, and findings for these outcomes were less consistent, with fewer attempts made to address non-causal explanations than for psychosis.
The authors say that recent estimates of the proportion of young adults and adolescents who have ever used cannabis is 40%. If having ever used cannabis increases the risk of a psychotic outcome by 41%, about 14% of psychotic outcomes in young adults in the UK would not occur in cannabis were not consumed.
The authors say: "We have described a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders."
They conclude: "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."
In the accompanying Comment, Drs Merete Nordentoft and Carsten Hjorthaj, Copenhagen University Hospital, Denmark say: "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."