Here's the truth about whether cannabis causes psychosis

When you have over 2 million people trying cannabis in the last year, the odds are that some of them already have or will go on to have mental health problems such as psychosis.

What has become clearer is that the more cannabis you smoke, the greater your risk of becoming psychotic – this is often referred to as the “dose-response” relationship. This means that a small group of people face the highest risk, as a mere 9 per cent of users get through 73 per cent of all the cannabis consumed. Also, for people who already have schizophrenia, cannabis can make the hallucinations and thought disorder they experience worse; they also take longer to recover from these when they do get treatment.

To get a sense of the size of the risk, we would need roughly 23,000 people to abstain from ever using cannabis to prevent one of them from becoming psychotic. That’s based on the research carried out to date. But the research isn’t perfect: one particular flaw stands out – most of it was conducted when people were using lower potency cannabis like resin.