Smoking cannabis virtually doubles the risk of developing mental illnesses such as schizophrenia, researchers say.
The study asked people about their cannabis use
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.
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.
The scientists, led by Professor David Fergusson, said it was likely cannabis use increased the chances of a person suffering psychosis by causing chemical changes to the brain.
Writing in Addiction, he added: "Even when all factors were taken into account, there was a clear increase in rates of psychotic symptoms after the start of regular use.
"These findings add to the growing body of evidence from different sources, all of which suggest that heavy use of cannabis may lead to increased risk of psychotic symptoms and disease in susceptible individuals."
Paul Corry, of the mental health charity Rethink, said: "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.
"Rethink is renewing its call to the Health Select Committee to investigate the latest research into the link between cannabis use and severe mental illness.
"We need action from the Department of Health and we need it now if we are to avoid the risk of tens of thousands of young people developing a severe mental illness in the future."
Marjorie Wallace, of the mental health charity SANE, said: "At last there is a convincing study supporting what we have been saying for many years, that there is a direct link between cannabis and psychosis.
"We urge the government to reconsider its decision on classification, backing that with a multi-million pound education and awareness campaign on the dangers of cannabis for young people whose brains are developing."
The Department of Health has said it will review all academic and clinical studies linking cannabis use to mental health problems.