Skunk, the strongest form of cannabis, could trigger psychosis, UK researchers warn.
In a study of 300 people, it was found that those who had suffered an episode of psychosis were 18 times more likely to have smoked skunk than other cannabis users.
A Royal College of Psychiatrists meeting heard they were also more likely to use cannabis every day.
Study leader, Dr Maria Di Forte from the Institute of Psychiatry at King's College London, said she looked at data from 197 people referred to a mental health unit with a first episode of psychosis, of whom 112 had used cannabis at some point and 120 controls, 72 of whom had used cannabis.
Among those who used the drug, people who had a psychotic episode were twice as likely to have used cannabis for longer, three times more likely to have used it every day and 18 times more likely to use skunk.
Skunk is three times stronger than other types and now accounts for between 70% and 80% of samples seized.
Dr Di Forte said if the preliminary results were proven it raised concern about the increasing availability of skunk.
She urged psychiatrists to question their patients more carefully about their drug habits.
"We should take a cannabis history in a more detailed way like we do when we take a history of cigarettes smoking to establish risk of lung cancer.
"But it would be naive to say that smoking a joint is safe as we do not have enough data to reach such conclusion," she added.
Co-researcher Dr Paul Morrison said skunk has higher levels of THC which causes the psychotic symptoms and lower levels of another compound called cannabidiol which seems to protect users from the effects of THC.
Tetrahydrocannabinol (THC) is the main psychoactive substance found in the hemp plant marijuana.
Richard Colwill, spokesman for the mental health charity SANE, said cannabis, particularly skunk, can be dangerous for the significant minority of people vulnerable to mental illness.
"We have daily evidence that it can trigger frightening psychotic episodes, relapse, and in some cases a life-long mental condition such as schizophrenia."
Cannabis is currently a class C drug but the Home Secretary has recommended it should be reclassified to a Class B drug because skunk now dominates the UK's cannabis market.
If approved by Parliament, reclassification would take effect from early 2009.
But experts urged caution over the interpretation of the findings.
The evidence on the link between cannabis and psychotic illness such as schizophrenia has been inconsistent.
Earlier this year the Advisory Council on the Misuse of Drugs concluded there was a probably a weak link between the two but that it is not yet clear if this link will become stronger as use of skunk becomes more widespread.