Marijuana users appear to be at higher risk of developing psychosis, British researchers say, although they could not pin the risk directly on marijuana use.

The Associated Press reported July 26 that marijuana users -- even those who just use the drug occasionally -- face up to a 40-percent greater risk of psychosis than nonusers. "The available evidence now suggests that cannabis is not as harmless as many people think," said researcher Stanley Zammit of Cardiff University, who drew his conclusions from a review of past research on marijuana and mental illness.

Heavy marijuana use appeared to raise the risk of psychosis between 50 and 200 percent, Zammit said.

The drug itself could be to blame, but Zammit left open the possibility that the increased psychosis risk could be related to users' "tendency to use other drugs or certain personality traits."

The review appears in the July 28, 2007 issue of the journal The Lancet.