May 1, 2007
One of the active chemicals in cannabis inhibits psychotic symptoms in people with schizophrenia, according to a study which compared it with a leading anti-psychotic drug. Although the finding could lead to new treatments for schizophrenia, scientists think it may also explain why cases of cannabis-induced psychosis are apparently on the rise.
Most cannabis research focuses on tetrahydrocannabinol (THC), the active ingredient that produces the high. Recent studies have shown THC makes symptoms of schizophrenia worse and triggers the condition in a small proportion of users.
But the new research shows that another chemical, cannabidiol (CBD), has the opposite effect. "One possibility is that there are good guys and bad guys within cannabis," said Markus Leweke, of the University of Cologne. He and his team compared the effects of CBD and a leading anti-psychotic drug, Amisulpride, on 42 patients with schizophrenia. After four weeks the symptoms of both groups had improved, but those treated with CBD suffered fewer side-effects.
"Maybe the cannabidiol ameliorates some of the effects of the THC and maybe it actually might be good for you if you are psychotic," said Robin Murray, of the Institute of Psychiatry at King's College London. They reported their research at the second International Cannabis and Mental Health Conference in London.
There is anecdotal evidence that the number of patients in the UK with psychotic symptoms linked to cannabis use is increasing. Professor Murray speculated that this may be linked to the increased THC content of herbal cannabis sold on the street. Cannabis on sale today has roughly doubled in strength in the last decade.
In a second set of experiments, researchers at the Institute of Psychiatry have shown how THC acts on the brain to induce paranoia. "By using brain scanning you can look in real time at the effect of the different components of the cannabis on healthy people and see how it's affecting the healthy brain," said Philip McGuire, a brain imaging expert at King's College London.
The team gave subjects a dose of THC or a placebo and then observed which areas of the brain were active while they performed a computer task that involved stopping certain movements. All volunteers were healthy men between 20 and 40 who had used the drug fewer than 15 times.
By looking at the difference in brain activity between placebo and THC groups, the researchers identified a region that is involved in controlling inappropriate behaviour. Those with the biggest reduction in the activity of that brain region also experienced more paranoid symptoms.
"One possible interpretation of that is the paranoia is being driven by interfering with this area of the brain," said Professor McGuire.