Professor Roger Pertwee, a neuropharmacologist at Aberdeen University, said it was well known that cannabis stimulated the appetite, but not widely known that the plant also contained substances that produced the opposite effect.

"We've discovered to our surprise that cannabis, as well as containing a drug that boosts appetite, contains a drug which has a blocking effect," he said.

The substance is called THCV and is chemically similar to another cannabis chemical, or cannabinoid, called THC that stimulates the appetite, he said.

Drugs based on THC are already being used to increase the appetite of Aids patients, and now the focus is on developing THCV for use as an anti-obesity drug, Professor Pertwee said.

Cannabis is rich in substances that can mimic the natural or endogenous cannabinoids in the brain that act as chemical messengers in the nervous pathways involved in such activities as appetite control or pain relief.

"These endogenous cannabinoids seem to act on the reward pathways to the brain, to increase the reward you get when you take food," he said.

"But ... they can have harmful effects. They can, for example, increase appetite and they can increase the laying-down of fat, which can give rise to obesity or overweight. Drugs are being developed that can increase the levels of these chemicals in our brains by slowing down the rate at which they disappear once they have been released," he said.

Professor Pertwee has also found a method of potentially boosting the signals in the brain that are generated by these endogenous cannabinoids.