Cannabis use increases the risk of psychosis later in life

Cannabis, also called marijuana, is the most commonly used illegal substance in most countries. In the developed world about 20% of young people report using cannabis at least once a week or heavy use, defined as use on more than 100 occasions. Most cannabis users start in adolescence, when the developing brain may be particularly susceptible to environmental exposure. There is evidence in the literature showing that cannabis intoxication produces transient, and usually mild, psychotic and mood-altering experiences. What this article is concerned with are long-term, chronic outcomes.

Theresa Moore and colleagues systematically reviewed long-term studies of cannabis use and any subsequent psychotic or mood-altering mental health outcomes. They also looked at how strong the evidence was in each of these studies.

They found that there is an increased risk of any psychotic outcome in people who have ever used cannabis and the greater the use, the greater the risk. Comment from Merete Nordentoft and Carsten Hjorthøj in the same edition of The Lancet points out that this is the most comprehensive meta-analysis to date of a possible causal relationship between cannabis use and psychotic and affective disorders later in life. One of the biggest problems with studies of cannabis that look at potential psychotic outcomes is that there is always the danger that it is the person's mental state that is causing cannabis use, rather than the other way round. This study adjusted for these potential confounding effects and made sure that psychosis was not simply the result of transient intoxication. Looking at statistics from the UK, Nordentoft and Hjorthøj surmise that, if there is a true causal effect, then 14% of psychotic problems in the UK would not occur if cannabis was not used. This could mean that around 800 cases of schizophrenia could be prevented if cannabis use decreased or stopped.

The ultimate test would, of course, be a large-scale randomised controlled trial of placebo against the active ingredient of cannabis, delta-9-tetrahydrocannabinol. But, cannabis is illegal in most countries and a harmful effect on cognitive function has already been established, so such a trial cannot be done.

This study finally brings into question the generally held view that cannabis is a relatively harmless drug compared with alcohol and other stimulants and opiods. Perhaps it is time to start looking seriously at the effects of all drugs of abuse – alcohol and tobacco included.

Moore THM et al. The Lancet 2007; 370: 319

For the abstract of this article, see here http://www.thelancet.com/journals/lancet/article/PIIS0140673607611623/abstract

For the full text of the editorial, see here http://www.thelancet.com/journals/lancet/article/PIIS0140673607611337/fulltext