The herbal supplement ginkgo biloba might be able to prevent some of the damage caused by a stroke, scientists report.

But they warn that it is far too early to recommend giving the supplement to stroke victims.

Much of the brain damage suffered during a stroke is caused by the release of so-called "free radical" molecules.

Ginkgo, made from the nuts and leaves of a tree, is a rich source of antioxidants - substances which it is thought can neutralise free radicals.

The supplement is marketed as a memory-boosting treatment, and widely sold in health food shops.

Researchers used a ginkgo preparation from Germany, and tested it on mice in which a stroke had been induced.

Low doses appeared to offer some protection against stroke damage - reducing the area of the brain affected by 30%.

However, larger doses were found to have no beneficial effects whatsoever.

This, and possible harmful side effects have led the research team to be cautious about recommending ginkgo to stroke victims.

Dr Wayne Clark, director of the Oregon Stroke Center and lead author of the report, said: "More work is needed to determine the proper dose.

"In addition, because ginkgo is also a mild blood thinner, it may be risky to use it in patients already on blood thinning medications prescribed in people at risk for stroke."

Unregulated supplies

Ginkgo contains a number of compounds which may have an effect on the body - the relative concentrations of these vary widely between different types of ginkgo supplement sold in the shops.

A spokesman for the Stroke Association echoed the advice of the researchers.

"The research has produced some very interesting results. However, as the researchers themselves make clear, they are a long way away from recommending the use of ginkgo in human beings.

"We agree that it is too early, but there are exciting possibilities."

Strokes happen when the flow of blood to a part of the brain is interrupted, usually by a clot of blood blocking a key blood vessel.

Starved of oxygen and nutrients, brain cells begin to die. When a portion of the brain is damaged, this can cause temporary or permanent disability.