How sturdy is the hyperlink between hashish and schizophrenia?

Not only is cannabis one of the world’s most widely used recreational drugs, it is also generating excitement for its potential to treat a range of health conditions from cancer to insomnia. But there is another side to the drug – it is associated with psychotic disorders, including schizophrenia.

Deepak D’Souza at Yale University has been studying this relationship for decades. He spoke to New Scientist about what his research, and the work of other scientists, has revealed – and why the link between cannabis and schizophrenia is so challenging to study.

Grace Wade: What do we know about the relationship between cannabis use and psychosis, specifically schizophrenia?

Deepak D’Souza: There are roughly three distinct relationships between cannabis use and psychosis. The first is that when some people consume cannabis, they experience an immediate psychosis-like episode with symptoms similar to schizophrenia. These can include delusions, hallucinations, blunted emotions and impairments in memory and speech. That psychotic episode is often sudden and dramatic. Sometimes it warrants going to the hospital. But it resolves fairly quickly, typically within hours or as soon as the drug wears off.

The second relationship is when psychosis occurs during cannabis use or soon after. Unlike the first scenario, this psychosis persists beyond the typical period of intoxication, for days or weeks, before resolving. It often requires some intervention like the temporary use of anti-psychotic medications.

The third relationship is when someone uses cannabis and then days, weeks, months or years later, develops a psychotic disorder. We…