So smoking ¨skunk¨cause psychosis, but milder cannabis doesn't?

The Mail on Sunday has shouted that ‘cannabis TRIPLES psychosis risk’ and that skunk is to blame for ‘1 in 4 of all new serious mental disorders’. Is this what the study they cite shows? Well, no, but it’s really interesting research which could advance our understanding of the relationship between cannabis and psychosis.

The research compares a group of first-episode psychosis patients in South London with a group from the general population in the same area. Not only did the authors ask participants about the frequency of their cannabis use, but also about the type of cannabis they used– specifically whether it was skunk, or hash.

They found that those with psychosis were much more likely to have used skunk every day, than to have never used cannabis. Conversely, people who smoked hash every day were no more likely to have psychosis than people who never tried cannabis.

But does this mean cannabis triples psychosis risk? The headline reported in the Mail came from a ‘population attributable fraction’ that the paper calculated: the number of cases of psychosis that would be prevented if all skunk use was removed, assuming that cannabis causes psychosis. The authors of the study estimated this to be 24%. However, the authors clearly point out that they cannot be sure the association seen in their study is causal.

There’s lots of consistent evidence associating cannabis use with psychosis and schizophrenia, but as the saying goes, correlation isn’t causation. For all sorts of reasons, you can’t randomly assign one group of teenagers to use cannabis, and another not to. This means you have to observe what people choose to do, and the people who choose to smoke cannabis might be different in a variety of other ways, which could be the cause of the increase in psychosis risk. Although you can control for these in analyses, you can never be sure you’ve adequately adjusted for them.

This study uses a case-control design. This type of study can lead to bias if the control population is not adequately selected. In this article, the controls were selected from people who lived in the same area of London. However, the authors noted some difference between the cases and controls, in particular those with psychosis were more likely to be male, more likely to be white Caucasian, and more likely to be heavy smokers. Psychosis is more common in men, and more common in ethnic minorities (in particular those of migrant status). The authors accounted for these differences and adjusted for other potential confounders, but there could be others that weren’t measured.

Read the full article.

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