Ginseng Reduces the Effect of Warfarin in a Study of Healthy Volunteers

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The summary below is from the full report titled "Brief Communication: American Ginseng Reduces Warfarin's Effect in Healthy Patients. A Randomized, Controlled Trial." It is in the 6 July 2004 issue of Annals of Internal Medicine (volume 141, pages 23-27). The authors are C.-S. Yuan, G. Wei, L. Dey, T. Karrison, L. Nahlik, S. Maleckar, K. Kasza, M. Ang-Lee, and J. Moss.

What is the problem and what is known about it so far?

Many people use both herbal therapies and prescription drugs, but little information is available about interactions between these 2 types of therapy. Warfarin is a commonly used anticoagulant drug. Doctors prescribe these drugs to prevent unwanted blood clots from forming in blood vessels or in the heart or to prevent existing unwanted blood clots from getting larger. Some people call anticoagulant drugs "blood thinners." Many things can influence the effectiveness of warfarin, including certain drugs, foods, and herbal therapies. Changes in warfarin's effectiveness have been reported in patients who were taking herbal therapies while receiving warfarin. Some people are concerned that ginseng, a popular herb, might interact with warfarin to decrease its ability to prevent clots, but no study has shown a ginseng–warfarin interaction.

Why did the researchers do this particular study?

To evaluate whether ginseng changed warfarin's effectiveness.

Who was studied?

20 volunteers in a research setting. The patients were not previously taking warfarin or ginseng. They were healthy and had no medical conditions that required treatment with warfarin.

How was the study done?

The study lasted for 4 weeks. During this time, the researchers gave the volunteers warfarin for 3 days during week 1 and 3 days during week 4. Beginning in week 2, the researchers assigned volunteers to take either ginseng or a placebo pill twice a day until the end of week 4. The placebo pills looked and tasted like the ginseng pills but contained no active ingredients. The researchers took blood samples from the patients to measure the effect of warfarin by using a test called the international normalized ratio, which measures clotting ability of the blood. They also measured blood levels of warfarin.

What did the researchers find?

After taking ginseng or placebo for 2 weeks, volunteers who were taking ginseng had lower blood levels of warfarin and less of an effect on blood clotting than those who took placebo.

What were the limitations of the study?

This study involved healthy volunteers but not patients who actually had medical conditions that required treatment with warfarin.

What are the implications of the study?

When prescribing warfarin, doctors should ask patients whether they use ginseng. Patients who take warfarin should be aware that ginseng can interfere with warfarin's effect, and they should tell their doctors that they use ginseng. Patients who take ginseng may require larger doses of warfarin to achieve its desired effects on blood clotting.