London, Nov. 22 (Reuters): Smoking cannabis increases the odds of suffering from depression and schizophrenia, doctors said today.

The occasional joint may not be harmful, but people who start using cannabis in their teens have a higher risk of later being diagnosed with a psychiatric disorder and the severity of the illness is linked to the length of exposure to the drug.

“Very young adolescents who use cannabis have an increased risk for developing schizoprehenia as adults and the most at risk are the youngest users,” Dr Louise Arseneault, of Kings College in London, told Reuters.

Doctors do not understand how cannabis increases the risk of schizophrenia and depression but they suspect it affects the dopamine system in the brain which is associated with pleasure. It is thought the drug can trigger the onset or relapse of schizophrenia in people predisposed to the illness and may also exacerbate the symptoms.

In a study of 1,037 people born in New Zealand between 1972-73, Arseneault found that those who began using cannabis as teens were four times more likely to suffer from psychiatric problems as adults than adolescents who did not use the drug. “The earlier you start, the more vulnerable you are,” she said.