Brain scans pinpoint cannabis mental health risk

"It's no longer a contentious issue. The expert community, by and large, accepts that cannabis contributes to the onset of psychotic symptoms in general and the severe form of psychosis, schizophrenia," he said.

DOUBLE-STRENGTH JOINTS

One reason for the growing problem is thought to be the increasing strength of modern strains of cannabis, which are cultivated to produce the maximum amount of THC.

In recent years, the average THC content of marijuana sold in Britain has doubled to 12 percent from around 6 percent, while in the Netherlands it is about 18 percent, Murray said.

Most users of cannabis still do not have a problem with the drug but a minority, possibly because of genetic factors, are vulnerable to long-term damage from modern skunk -- which Murray says is to old-fashioned dope what whisky is to lager.

The rise in THC content is linked with a decline in another active ingredient called cannabidiol (CBD), since the two products compete biochemically inside the cannabis plant.

CBD, which reduces anxiety but does not produce the euphoric high of THC, may help offset some of the paranoid feelings.

Markus Leweke of Cologne University said a clinical trial involving 42 patients showed CBD was as effective as the established medicine amisulpride, sold as Solian by Sanofi-Aventis, in treating patients with psychosis.

"It seems there are good guys and bad guys within cannabis," Leweke said.