A Zurich University study claiming to establish a link between smoking cannabis and developing schizophrenia has sparked debate in Switzerland.

Researchers found an increased rate of schizophrenia among young people during the liberal 1990s. But how many of these patients actually used cannabis remains unknown, critics point out.

The study analysed data from around 8,000 male and female patients in canton Zurich who were first-time admissions for schizophrenia between 1977 and 2005. The 1990s showed a higher incidence of the condition in the age groups most likely to use cannabis.

The findings showed that males in the 15 to 19-year-old age group were three times more likely to develop the illness in the 1990s than in other decades and that susceptibility doubled for 20 to 24-year-olds, according to the research which was published on Monday.

Study co-author Wulf Rössler told swissinfo that the results proved a direct link to cannabis usage.

"We know from other experimental studies that cannabis can cause psychosis, but we have now established a clear link to schizophrenia for the first time," he said.

"The risk of developing schizophrenia has a direct relationship with the rate of consumption. With occasional users it does not increase the risk, but if you smoke regularly, daily, over a period of years, then it increases the risk two to three times."

Not convinced

But the Federal Health Office is not convinced by the report, pointing to another unexplained leap in schizophrenia in the mid-1980s and the fact that the patients' drug histories and other medical details remained unknown.

"It does not uncover the medical history of the patients, for instance the consumption of psychotic substances or other factors that could lead to psychotic illnesses," said the health office in a statement.

"The connection between schizophrenia and cannabis consumption is not clarified yet."

Ambros Uchtenhagen, an expert in drug issues at the Zurich-based Institute of Social and Preventative Medicine, welcomed the report, but also sounded a cautious note about its findings.

"The results are purely hypothetical and they should not be misused in any way, particularly for political purposes to say that we now know that cannabis produces schizophrenia," he told swissinfo.

Warning issued

"Nobody knows if these people have ever used cannabis. This is an interesting hypothesis and it is an invitation to look more closely at what happened."

The Swiss Institute for the Prevention of Drug and Alcohol Problems said it found the hypothesis "quite conceivable", but not proof, and pointed to other research that suggested a connection between cannabis and schizophrenia.

Spokeswoman Gerlind Martin also warned vulnerable people against dabbling in drugs.

"Young people who are in their developmental stages and adults experiencing difficult life situations should not consume drugs, including cannabis," she said.