Cannabis, the third most popular recreational drug after alcohol and tobacco, yesterday won an unlikely accolade from scientists who said that it could boost brain power.

Cannabis, the third most popular recreational drug after alcohol and tobacco, yesterday won an unlikely accolade from scientists who said that it could boost brain power.

Cannabis, the third most popular recreational drug after alcohol and tobacco, yesterday won an unlikely accolade from scientists who said that it could boost brain power.

Experiments on rats given a potent cannabinoid have shown the drug stimulates the growth of new brain cells. Canadian researchers found that the drug caused neurons to regenerate in the hippocampus, an area that controls mood and emotions, after one month of treatment.

Its effect was similar to that of the antidepressant drug Prozac, which also stimulates nerve growth in the hippocampus. The rats were less anxious and more willing to eat in a novel environment that would normally make them fearful.

Most drugs, including alcohol, heroin, cocaine and nicotine, have been shown to destroy nerve cells in the hippocampus, the researchers from the University of Saskatchewan, Canada, say. "The present study suggests that cannabinoids are the only illicit drug that can promote adult hippocampal neurogenesis following chronic administration," they write in the Journal of Clinical Investigation.

The finding runs counter to previous research highlighting the risks of cannabis use, including a heightened degree of psychosis in vulnerable users, and an increased risk of lung cancer similar to that in tobacco smokers. The authors say regular cannabis users are known to suffer acute memory impairment, as well as dependency and withdrawal symptoms.

The new research suggests that the size of the dose may be crucial. The results showed that regular injections of high, but not low, doses of the artificial cannabinoid HU210 were associated with anti-anxiety and antidepressive effects.

"These complicated effects of high and low doses of acute and chronic exposure to cannabinoids may explain the seemingly conflicting results observed in clinical studies regarding the effects of cannabinoid on anxiety and depression," the scientists say.

The study emerged from the recent discovery that, unlike other parts of the brain, the hippocampus can generate neurons throughout the lifespan of mammals, including humans.

Natural selection has conserved cannabinoid receptors in animals that have been separated by evolution for 500 million years, suggesting they have an important biological role. Cannabinoids appear to alter the effects of pain, nausea, tumours, sclerosis and other disorders in both animals and humans, the team says.