Cannabis users get schizophrenia younger

BMJ Group News

People with symptoms of schizophrenia will typically have started getting their symptoms two years earlier if they have used cannabis, according to new research. The study adds to the evidence that cannabis may increase the risk of this mental illness.

What do we know already?

There’s been a lot of research and discussion around whether smoking cannabis - especially strong cannabis or skunk - increases the chances of mental illness, in particular schizophrenia.

Schizophrenia is characterised by episodes where the affected person loses contact with reality, described as psychosis. The person might very firmly hold delusional beliefs that can’t possibly be true, or hallucinations, such as hearing voices or seeing things that aren’t really there.

We know that more people with psychosis use cannabis compared to people without mental health problems, although that’s not proof that cannabis causes psychosis. People with mental health problems might be more likely to use cannabis to attempt to ease the problems caused by their illness.

However, if research shows that people who smoke cannabis get psychosis younger than other people, that adds weight to the argument that the drug might cause the illness (although it’s not conclusive proof). A new study has tried to find out.

What does the new study say?

All the people in the study had been diagnosed with schizophrenia. Those who’d used cannabis had their first episode of psychosis on average two to three years younger, compared with people with schizophrenia who hadn’t used cannabis.

For comparison, alcohol use made no difference to the age at which they first got psychosis.

How reliable are the findings?

The study was a summary of all the published research into this topic (a meta-analysis). The researchers pooled the results of the other studies to come up with their overall result. That can be a good way of getting enough data to find a reliable result. However, it depends on the quality of the studies they used.

Where does the study come from?

The study was done by researchers in Australia and the US. It was published in the Archives of General Psychiatry, which is owned by the American Medical Association.

What does this mean for me?

This study adds to the evidence that young people in particular may be at risk of developing schizophrenia if they regularly use cannabis. But the research is not conclusive.