A study confirms fears that using cannabis can increase the risk of developing psychotic illnesses later in life.
Cannabis is the most commonly used illegal substance in the UK and around 20% of young people report smoking it at least once a week.
Dr Stanley Zammit and colleagues at Bristol University analysed 35 studies and assessed the relationship between cannabis and mental health disorders.
Individuals who used cannabis were 41% more likely to develop any type of psychosis than those who had never used the drug.
The risk of psychosis increased relative to the dose used, with more frequent cannabis users more than twice as likely to develop mental health problems.
But senior author Professor Glyn Lewis at the University of Bristol said it is difficult to judge whether cannabis causes schizophrenia as the user may have other characteristics that predispose them to psychotic illness.
Although he adds: "All the studies have found an association and it seems appropriate to warn members of the public about the possible risk."
Dr Zammitt commented that even if cannabis dose increase the risk of developing psychosis, most people will not be affected.
"Nethertheless, we would still advise people to avoid or limit their use of this drug, especially if they start to develop any mental health symptoms or if they have relatives with psychotic illnesses." he says.