BBC NEWS | Health | Drug 'doubles mental health risk':
The study asked people about their cannabis use
Smoking cannabis virtually doubles the risk of developing mental illnesses such as schizophrenia, researchers say.
The New Zealand scientists said their study suggested this was probably due to chemical changes in the brain which resulted from smoking the drug.
The study, published in the journal Addiction, followed over 1,000 people born in 1977 for 25 years.
UK mental health campaigners said it was more evidence of a 'drug-induced mental health crisis'.
The researchers, from the University of Otago, interviewed people taking part in the Christchurch Health and Development Study about their cannabis use at the ages of 18, 21 and 25.
This is the latest in long line of international research over the last 12 months that shows we are facing a drug-induced mental health crisis
Paul Corry, Rethink,
They were also interviewed about various aspects of their mental health.
The scientists found psychotic symptoms were more common among cannabis users.
They analysed their findings to take into account the possibility illness encouraged people to use more cannabis, rather than the drug contributing to their condition.
But the researchers said the link was not likely to be due to people with mental illness having a greater wish to smoke cannabis.
Instead, they said cannabis may increase the chances of a person suffering psychosis by causing chemical changes to the brain.
The researchers also took into account factors such as family history, current mental disorders, and illicit substance abuse."