Science / Medicine : A Weekly Roundup of News, Features and Commentary : Pot Linked to Schizophrenia

A 15-year study of more than 45,000 Swedish soldiers suggests that heavy marijuana users are six times more likely than non-users to develop schizophrenia.

The authors of the study said the statistical association between schizophrenia and marijuana does not necessarily mean that the drug causes schizophrenia.

“Cannabis consumption might, on the contrary, be caused by an emerging schizophrenia,” the researchers said in a report in Lancet, a British medical journal.

Dr. Charles Schulz, chief of pharmacology research for the schizophrenia branch at the National Institute of Mental Health, said the findings are not surprising. “Many psychiatric clinicians have noted over the years that a patient’s illness starts with marijuana use or overuse,” he said, explaining that marijuana might cause latent schizophrenia to become apparent.

Researchers have also noted that the hallucinogenic drug LSD can trigger persistent mental illness, Schulz said.

Furthermore, there is a chemical link that might explain why marijuana and schizophrenia would be related, Schulz said.

In the Swedish study, 45,570 soldiers were given questionnaires at the time of their induction into the country’s compulsory military service. They were asked about drug use, among other things. They were also given psychiatric evaluations.

The researchers found 197 cases of schizophrenia among 41,280 members of the group who reported no marijuana use. In contrast, 21 cases of schizophrenia were found in the 752 soldiers who reported heavy marijuana use, defined as use on more than 50 occasions.

From those figures, the researchers calculated that the heavy marijuana users had six times as great a risk of schizophrenia as the non-users.