STUDY: Daily marijuana use increases risk of psychotic disorder

The legalisation of medical-use marijuana continues to sweep across the globe, recently in Thailand where the roll-out and enactment of practical uses of the new legislation are underway.

But this spread of a new legal credibility of the drug continues whilst possible health risks (or benefits) are not fully understood. Properly medically supervised or scientifically conducted studies continue to come out weekly with varied results about the benefits or dangers of long-term cannabis use.

According to new research published in ‘Lancet Psychiatry‘, a weekly peer-reviewed general medical journal, people who use cannabis daily, as well as those who use high-potency weed, may be three times more likely to develop psychotic disorder than never-users.

The new evidence is consistent with previous experiments that suggest heavy use and high THC concentration cannabis – a 10% concentration of THC (the psychoactive substance within cannabis) or higher – can be harmful to mental health.

Dr. Marta Di Forti, lead author and a clinician scientist at the Institute of Psychiatry, Psychology & Neuroscience, King’s College London said the ‘Psychotic disorder’ was precisely what was studied.

“We are talking about people who meet diagnostic criteria and come to the attention of mental health services to receive treatment for psychosis. So they have to have symptoms of psychosis across the spectrum – hallucination, delusions – that have lasted at least for a week.”

Currently, medical cannabis is legal in most European countries, though recreational use is only legal in Netherlands, Czech Republic and Spain (in certain situations). Meanwhile many other countries continue to discuss legalisation.

Di Forti and her co-authors pf the paper looked at data from five countries in Europe… UK, the Netherlands, Spain, Italy and France. Brazil was also included in the sample where cannabis is illegal.

They found 901 patients with a first-time episode of psychosis over a five-year period and compared them to 1,237 matched non-patients.

Daily use of cannabis was more common among patients with psychosis compared to the controls, they found. About 30% of patients reported using cannabis daily compared to just 7% of non-patient controls. And use of high potency cannabis was also more common among patients than controls – 37% compared to 19%.

The study results do not provide enough information for her to say “use only this amount, only this often” to remain safe.

At this stage, the paper estimates one in five new cases of psychosis may be linked to daily cannabis use, and one in 10 cases linked to use of high potency cannabis.

You can read the full report in Lancet Psychiatry.