LONDON (Reuters) - Brain scans showing how cannabis affects brain function may help explain why heavy consumption of the drug triggers psychosis and schizophrenia in a small number of people, scientists said on Monday.

An unidentified man rolls a marijuana joint in a cafe in Rotterdam Jun 20, 2005. Brain scans showing how cannabis affects brain function may help explain why heavy consumption of the drug triggers psychosis and schizophrenia in a small number of people, scientists said on Monday. REUTERS/Jerry Lampen JFL/KY

Psychiatrists are increasingly concerned about the mental health impact of smoking large amounts of modern super-strength marijuana, or skunk, particularly among young people.

Until now, the mechanism by which cannabis works on the brain has been a mystery but modern scanning techniques mean experts can now detect its impact on brain activity.

Professor Philip McGuire and Zerrin Atakan of London’s Institute of Psychiatry said their work using magnetic resonance imaging, or MRI, showed patients given the active cannabis compound tetrahydrocannabinol (THC) had reduced function in the inferior frontal cortex brain region.

This area is associated with controlling inappropriate emotional and behavioural responses to situations.

“What THC seems to be doing is switching off that part of the brain, and that was associated with how paranoid people became,” McGuire told reporters.

Their research will be presented at a two-day International Cannabis and Mental Health Conference at the Institute of Psychiatry this week.

Similar findings from other teams also highlight the link between THC dose and the risk of schizophrenia-like symptoms, conference organiser Professor Robin Murray said.

“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.