The findings are significant given that supplement users tend not to tell their doctors if they are taking supplements. Furthermore, ginseng is one of the best-selling supplements in many markets, with Mintel reporting a massive 57 rise in sales of ginseng in the UK during 2003.

However labels on supplement products do not always inform consumers of the potential interactions with prescription drugs, particularly if they are registered under food law as food supplements - the case with ginseng in the UK.

But while the medical community pays increasing attention to growing herbal use and potential drug interactions, two factors make it difficult for supplement makers to improve consumer awareness of these risks.

"We include a general statement on all our products advising consumers to check with their doctor before taking herbals. But it is difficult to be specific when we don't have enough research. The majority of herbs have not been tested against the majority of drugs," Paul Chamberlain, director of technical affairs at Solgar UK.

"This is really an emerging area of research and as more is done, there will be better information available."

"But this is also a symptom of the way such products are being sold," he added. "Most herbals in the UK are sold under an exemption of medicine law as foods, and therefore there is no real requirement to put warnings on the labels."

He noted that Solgar sends an education team to health food stores distributing its products to train staff there on the actions and potential interactions of products.

"We're very aware of this issue and part of our distribution policy is to sell in outlets where consumers are informed. At the same time, we hope it works the other way round and drug makers start adding information on these interactions on their labels too. Many more people are taking drugs than herbals after all."

Last year King's College researchers told a British Pharmaceutical Conference that one in 20 people are taking potentially dangerous combinations of herbal and prescription medicines.

The most common herb drug interaction was St Johns Wort with selective serotonin re-uptake inhibitors, taken by 30 per cent of those surveyed, said the researchers. Other potentially dangerous interactions include taking gingko with aspirin, which can lead to haemorrhage and taking St Johns Wort with the contraceptive pill, which can cause nausea, vomiting, headaches and anxiety.

However scientists have also underlined that most of the research into herb/drug interactions offers poor evidence of the effects, with many of the studies too small or not properly carried out.

In the new study, published in today's issue of the Annals of Internal Medicine, Chun-Su Yuan, director of the Tang Center for Herbal Medicine Research at the University of Chicago, said he gave 20 healthy subjects 5 mg of warfarin daily for three days during week one and again in three weeks later.

Beginning in week two, 12 subjects took two grams of powdered ginseng in capsules. The other eight volunteers received a placebo. The researchers monitored blood levels of warfarin and the clotting ability of the blood. They found that after two weeks, daily doses of ginseng significantly reduced the blood levels and the anti-clotting effects of warfarin.

Since ginseng alone can promote bleeding and delay clot formation, the researchers were surprised to find that it reduced the anti-clotting effect of warfarin compared to those who took the placebo. They suspect that substances within ginseng may enhance the function of enzymes that break down warfarin, clearing it from the blood stream more rapidly.

"Warfarin has a narrow therapeutic index," said Yuan, "which means precise dosing is crucial."

"With too small a dose, the risk of clots increases, but too much can cause serious bleeding. So a substance, such as ginseng, that alters warfarin's effects, even slightly, can have significant consequences," he added.

Warfarin is used to prevent blood clots from forming or growing larger. It is often prescribed for patients with certain types of irregular heartbeat, those who have had a heart attack or undergone heart valve replacement surgery. It is the most common oral anticoagulation therapy and its use is growing. Around 750,000 people in the UK were thought to be using the medication in 2001.

Cranberry juice, thought to benefit heart health, has also been found to interact with the blood-thinning drug.