Using marijuana increases the risk of one day developing a psychotic illness such as schizophrenia, according to a study that provides some of the strongest evidence yet linking the drug to a mental disorder.

Marijuana is one the most commonly used illegal substances in many countries with up to 20 per cent of young people in places like Britain reporting either some use or heavy use, British researchers said, citing government statistics.

Many consider it on par with alcohol or tobacco but the results shows marijuana poses a danger many smokers underestimate, said Stanley Zammit, a psychiatrist at Cardiff University and the University of Bristol, who worked on the study.

The researchers found that marijuana users had a 41 per cent increased chance of developing psychosis marked by symptoms of hallucinations or delusions later in life than those who never used the drug. The risk rose with heavier consumption.

"If you compare other substances like alcohol or tobacco it may not be as harmful, but what we are saying is neither is it completely safe," Dr Zammit said in a telephone interview. Other findings have highlighted the link between marijuana use and the risk of schizophrenia-like symptoms such as paranoia, hearing voices and seeing things that are not there.

But this study marks one of the most comprehensive, thorough and reliable reviews of its kind and should serve as a warning, two Danish researchers wrote in an accompanying comment in the Lancet medical journal, which published the study yesterday. They said the results mean an estimated 800 cases of schizophrenia in the United Kingdom could be prevented each year by ending marijuana consumption.

"We therefore agree with the authors' conclusion that there is now sufficient evidence to warn young people that cannabis use will increase their risk of psychosis later in life," they wrote.