THC, the main active ingredient in marijuana, can trigger temporary psychosis in some people, but another compound in the drug may ease psychiatric symptoms among schizophrenics, researchers say.

The Associated Press reported May 1 that a study involving brain scans of 15 test subjects given doses of THC found that the substance produced hallucinations and paranoid delusions. The brain scans showed that THC interfered with the inferior frontal cortex of the brain, a region known to be associated with paranoia.

"THC is switching off that regulator," said researcher Philip McGuire of King's College in London. "We've long suspected that cannabis is linked to psychoses, but we have never before had scans to show how the mechanism works."

In another study, Yale University researchers said that THC made psychotic symptoms worse for schizophrenics, even though test subjects said the drug calmed them down. "I was surprised by the results," said researcher Deepak Cyril D'Souza of the Yale University School of Medicine. "In practice, we found that cannabis is very bad for people with schizophrenia."

"One of the great puzzles is why people with schizophrenia keep taking the stuff when it makes the paranoia worse," said Robin Murray, professor of psychiatry at King's College.

A possible answer can be found in a study of cannabidiol (CBD), another ingredient of marijuana; this study found that CBD inhibits psychotic symptoms among schizophrenics, the Guardian reported May 1.

"One possibility is that there are good guys and bad guys within cannabis," said researcher Markus Leweke of the University of Cologne, who compared the effects of CBD with the anti-psychotic drug Amisulpride.

"Maybe the cannabidiol ameliorates some of the effects of the THC and maybe it actually might be good for you if you are psychotic," added Robin Murray, of the Institute of Psychiatry at King's College London.