A UK study that attempted to untangle the casual factors behind schizophrenia and the role of cannabis, has suggested that patients with the condition use the drug as self-therapy.

The researchers found a causal link between trying cannabis and an increased risk of the condition, but noted that there was stronger evidence that patients at risk of developing schizophrenia were more likely to try the drug.

“There was strong evidence consistent with a causal effect of schizophrenia risk on likelihood of cannabis initiation” Study authors

The new study, by the University of Bristol, follows warnings in the national press from scientists about the increased risk of psychosis for vulnerable people using cannabis.

Those warnings, made in April, themselves followed evidence to suggest an increased use of particularly high potency strains of cannabis among young people, often known as “skunk”.

However, experts cautioned that the risks should not be overstated, given the need for greater research into links between mental health and illicit drugs.

The Bristol study sheds fresh light on the issue, though the authors cautioned that their results should still be considered in the wider context of other mental health factors.

While some evidence was found to support hypotheses that cannabis use is a contributory factor in increasing the risk of schizophrenia, the researchers said they were surprised to find stronger evidence that the opposite was also likely.

It added weight to the idea that the drug may be used as a form of self-medication by patients with the condition, they suggested.

The researchers used a new research technique – using genetic variants that predict either cannabis use risk or risk of developing schizophrenia – to examine data from earlier studies.

The technique was used as an alternative to traditional observational epidemiology in an attempt to account for other variants that may affect the association, given that cannabis users are likely to be different from those who do not in lots of other ways.

Writing in the journal Psychological Medicine, the study authors said: “There was some evidence consistent with a causal effect of cannabis initiation on risk of schizophrenia. There was strong evidence consistent with a causal effect of schizophrenia risk on likelihood of cannabis initiation.”

“It seems that heavy cannabis use is most strongly associated with risk of schizophrenia” Suzi Gage

Study author Dr Suzi Gage said: “The evidence suggested that schizophrenia risk predicts the likelihood of trying cannabis. However, the relationship could operate in both directions.

“Our results don’t really allow us to accurately predict the size of the effect – they’re more about providing evidence that the relationship is actually causal, rather than the result of confounding or common risk factors,” she said.

Dr Gage added: “While we find stronger evidence that schizophrenia risk predicts cannabis use, rather than the other way round, it doesn’t rule out a causal risk of cannabis use on schizophrenia.

“What will be interesting is digging deeper in to the potential sub-populations of cannabis users who may be at greater risk, and getting a better handle on the impact of heavy cannabis use,” she said.

She added: “What would really help progress this research is to use genetic variants that predict heaviness of cannabis use, as it seems that heavy cannabis use is most strongly associated with risk of schizophrenia.”