A British advisory group this year found no evidence to reclassify cannabis as a more harmful drug in that country. In contrast to the U.S., the U.K. puts cannabis in the lowest category (Class C) in terms of criminal penalties for possession or sale, although government officials are campaigning to move it to Class B.
To investigate the risks of marijuana, researchers typically use heavy marijuana smokers as subjects. Though such a study design may be convenient, it makes interpretation tricky because heavy users may have traits in common besides smoking pot. Thus, says psychologist and marijuana researcher Stanley Zammit of Cardiff University in Wales, it is not easy in these kinds of studies to separate out the contribution of marijuana to any measurable effect in the group.
Psychosis
Claims of a link between marijuana use and psychotic episodes came under scrutiny after the U.K. downgraded cannabis from Class B to Class C in 2004. In 2007, Zammit was asked by England's Department of Health to survey the existing evidence to determine the long-term risks for mental illness from using cannabis. After researching the literature and including only those studies that satisfied certain criteria, he combined the results in a 2007 Lancet paper.
He concluded that marijuana use was associated with an increased risk of psychosis -- ranging from self-reported symptoms such as delusions or hallucinations to clinically diagnosed schizophrenia.
The risk is small, he adds. Cannabis use was associated with a 40% increase in risk overall and up to a twofold increase in heavy users. Because the risk of any person developing psychosis in their lifetime is about 2% to 3%, cannabis use at worst increases that to 5%. "So 95% of the people are not going to get psychotic, even if they smoke on a daily basis," Zammit says.
Zammit adds that "the main limitations of these studies is that you can never be sure that it's the cannabis itself that's causing this risk." Heavy users of marijuana may differ from nonusers in other traits -- including those that lead independently to increased drug use and risk of psychosis. The studies he reviewed tried to take into account this possibility but could not rule it out entirely.
The bottom line? "The evidence is probably strong enough that people should be aware of this risk," he says.
