Some cannabis-using teenagers are highly likely to develop mental health problems as adults because of their genetic make-up, but most face little risk, a new study says.

The problems occur for genetically vulnerable adults who used cannabis while their brains were still developing, and that has prompted researchers to recommend people "delay onset of use until at least young adulthood".

The finding comes from Otago University's world-renowned study which has followed 1000 Dunedin-born people for over 30 years.

It says a link between cannabis and mental illness depends on a specific genetic disposition. But even then, only 15 per cent of the people with the gene will develop cannabis-related psychotic behaviour, and only if they use it as adolescents.

Other studies have identified a connection between cannabis and psychotic behaviour but this research - a combined effort by Otago University, London's King's College and the University of Wisconsin - has taken it a step further by pinpointing the exact gene.

The study repeatedly questioned 803 participants in the Dunedin Multidisciplinary Health and Development Study about their cannabis use when aged 13, 15 and 18.

When they were 18, 27 per cent said they used cannabis at least once a month, some every day. By the time they were 26, 3 per cent overall had developed some recognised psychosis: 1 per cent had schizophrenia and 2 per cent had milder conditions.

The study defined psychosis as "a period of more than a month in the past year when the individual constantly experiences hallucinations, delusions, loss of pleasure, low motivation for normal activities and disorganised behaviour".

Researchers questioned the 803 study members, and their family and closest friends.

Among those in the study who used cannabis as teenagers, the rate of psychosis was 8 per cent. But when broken down further to those who had the cannabis-vulnerable gene, the rate of psychosis was 15 per cent.

The gene is called COMT and everyone has two copies - one inherited from each parent - which then develop into one of three different combinations. The cannabis-vulnerable combination occurs in 25 per cent of the population.

One of the researchers, Associate Professor Richie Poulton of Otago, says the findings carried an important message for a small part of the population.

"We are talking about a very real increase in risk but it is constrained to the minority of people during a particular period of their development."

People should not be alarmist about the findings or "interpret this as supporting extreme views either that cannabis is extremely harmful or the reverse.

"Neither position is supported by the facts and people that promulgate those ideas do the youth of any country a disservice".

In terms of the total population, he said cannabis-related mental illness was not a major health risk.