Don't make a hash of it: the link between cannabis and psychosis

The long-suspected link between consuming cannabis and developing schizophrenia has been repeatedly confirmed by recent studies” claimed Patrick Cockburn of The Independent at the end of this November.

This newly revived and ostensibly persuasive rhetoric concerning the causal link between cannabis use and psychosis related disorders has become popular in the media over the last five years. However, we must be very cautious when interpreting these sometimes hastily drawn inferences.

All of the data that the media latch on to in order to champion the dangers of cannabis use is correlational. It simply looks at the relationships between variables in the real world because an experimental investigation into cannabis use and psychosis would be ethically dubious at the very least.

Correlational research suffers a host of problems. Even when a relationship between cannabis use and schizophrenia is found, it is difficult to negate the multitude of other factors that may be involved in the development of the disorder.

Nonetheless, this research must be considered, as it is the only data available. For instance, a Swedish study with over 50,000 army conscripts found that people who had smoked cannabis before the age of 18 are 2.4 times more likely to develop schizophrenia. Similarly, a 2002 paper found that participants who smoked cannabis before the age of 15 are four times more likely to develop schizophrenia. On the face of it then, it seems that adolescent cannabis use is causing the development of psychosis related disorders.

Why is this conclusion erroneous? It ignores the existence of the many inter-related factors. Maybe schizophrenic people smoke more cannabis in their early years because their disorder is emerging and they need to self-medicate with cannabis. Maybe it’s not cannabis that is contributing to the development of the disorder, but other related variables, like urban living and an unstable family life. Maybe the huge increase in online gaming and isolated lifestyles is a critical factor. Cannabis use probably does have a small contributory effect, but this must be thought about like all of the other contributory factors, rather than the sole cause of the disorder, as the media likes to portray it.

This is not to say that these relationships should be ignored. In fact, with the ever-growing accumulation of data pointing towards the relationship it needs to be seriously considered, especially when drug policy is likely going to change. Yet it needs to be considered in sensible ways.

Importantly, this data should not be used to support an anti-legalisation argument. If anything, this putative relationship adds to the argument in favour of drug policy reform. The THC content in street cannabis has sky rocketed over the last few decades, moving from around 1-5% to up to 20%, while the content of CBD, which has anti-psychotic effects, has fallen.

A 2008 study from UCL found that people who smoke cannabis with a significant CBD content did not show any more psychotic symptoms than non-users. Whereas, those who smoked cannabis with no CBD showed significantly more psychotic symptoms. If the government had control over the drugs sold to the public, as they do with alcohol and tobacco, the THC and CBD content could be carefully moderated, and could reduce the already small association between cannabis use and schizophrenia.

How much does cannabis use even really contribute to the amount of schizophrenia sufferers in our population? 1-2% of people suffer this disorder, and a high estimate of cases that are linked to cannabis use is 7%. So only 0.14% of people develop schizophrenia that is related to cannabis use. And it must be remembered that “related” means only that it has a contributory effect, like the numerous other risk factors, something sensationalist media articles consistently fail to do.

So what should be made of all of this? It would be foolish to deny that there is not a link between cannabis use and schizophrenia. In fact, it seems likely, although one cannot be certain, that cannabis use contributes in a small way to these disorders’ development. Yet this association is small and must be considered with all of the other risk factors in mind. If you’re going to get seriously worried about cannabis use causing schizophrenia, then you better start getting worried about pregnant mothers having the flu, a child’s parents divorcing, and living in the city rather than the countryside. These all increase the chances of schizophrenia, yet the media manages to get over-excited simply because cannabis is an illegal drug. So let’s be realistic and make sure the government doesn’t make a hash of this moderate association when the legal status of cannabis is inevitably discussed.