Extracts of the marijuana plant could one day be routinely used to prevent brain damage after stroke, according to United States government scientists.
A team led by the British-born biologist Aidan Hampson, at the US National Institute for Mental Health, in Maryland, has discovered that two active components of cannabis - compounds called THC and cannabidiol - will each act to prevent damage to brain tissue placed in laboratory dishes.
The experiments, to be reported next week in the proceedings of the National Academy of Sciences, reveal an unexpected potential use for a drug known for centuries to have valuable medical properties. The discovery is likely to increase pressure to make marijuana and its derivatives more widely available for use on prescription.
Already, a House of Lords committee is considering the issue, the British Medical Association has reported on the drug's virtues and the Royal Pharmacological Society is looking into the matter.
Cannabis was widely used centuries ago. There is archaeological evidence from the Stone Age of cannabis being used to ease birth pains. It is known to suppress nausea for patients on cancer chemotherapy, relieve pain and muscle spasm for multiple sclerosis sufferers, and reduce pressure in the eye for people with glaucoma.
Dr Hampson's study has focused on cannabidiol, rather than the psychoactive chemical THC, because this substance has no side-effects. He stumbled on the finding while trying to find out why the human brain had so many "receptors" for cannabis compounds and what the receptor system was designed to do.
"There are almost as many cannabinoid receptors as there are of any major neurotransmitter, so while no one knows what it does, it seems to be pretty important."
Stroke victims suffer a blood clot which starves brain cells of glucose and oxygen, and sets off a cascade of chemical reactions which destroys cells. He found that both cannabis compounds seemed to block the destructive process. Some drugs work well in test tubes, but ail in living creatures because they do not reach the target. Cannabis compounds go straight to the brain.
The results suggest that cannabidiol could also become a treatment for other neurological disorders, such as Parkinson's and Alzheimer's diseases. Dr Hampson said: "We have something that passes the brain barrier easily, has low toxicity, and appears to be working in the animal trials. So I think we have a good chance."