Cancer: Herbs can interfere with chemotherapy, doctors caution

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Mar 27, 2005
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Avoid mixing supplements, cancer treatment
Herbs can interfere with chemotherapy, doctors caution

By Karen Collins, R.D.
Registered Dietitian
Special to

Updated: 9:40 a.m. ET March 11, 2005

Surveys show that many cancer patients turn to herbal supplements hoping to decrease the side effects of treatment or boost the body's ability to defeat cancer. For many years, there was no sound research on which to evaluate the effectiveness and safety of these supplements - patients just took their chances.

Although there are still a lot of questions about herbal supplements, experts now say there is some basis for evaluating them. If you are a cancer patient considering one or more of these products, it is critical that you tell your doctor.

Herbs have the reputation of being natural and gentle, but they contain compounds that can profoundly affect the body. Research continues on how to harness their bioactivity to make more effective or more tolerable cancer treatments. The strength that herbals may have for positive effects could significantly interfere with treatments if the wrong products are used at the wrong times.

There is another misconception about herbal products, and all supplements for that matter, which everyone should be aware of. Consumers may not always attain the same benefits observed in research studies, because supplements may differ from what was used in research.

Independent tests on a variety of herbal products have shown that some are missing significant levels of important compounds, or are contaminated with unlisted ingredients. One safeguard against this problem is to check the label for a mark by the United States Pharmacopeia (USP) that says “dietary supplement verified” in a circle. For a partial list of supplements verified by USP for quality you can visit Another organization that independently tests the quality of supplements is website is

Unsupported claims
Several reports highlight the problems that a use of herbs can cause during cancer treatment. For instance, depression and anxiety are common after a diagnosis of cancer. But when people take St. John's Wort as a remedy, this product can interfere with the actions of certain chemotherapy drugs, decreasing their effectiveness. Furthermore, this herb stays in the body for several weeks.

Another common reason for using herbs is to deal with side effects of treatment. Ginger is widely used for nausea. But both ginger and garlic, when used in large amounts as supplements rather than the small amounts used in flavoring food, can slow blood-clotting times. Especially for someone using one of the chemotherapy drugs that also slows blood clotting, serious bleeding problems can arise.

A third reason people use herbs or supplements is to boost their immune systems. Echinacea is widely used for this purpose. But studies suggest that after eight weeks, echinacea may actually suppress the immune system and damage the liver. Another herbal remedy, essiac tea, combines several herbs and is said to strengthen the immune system and relieve pain. So far no human clinical trials have been completed to support these claims. Past animal studies did not verify them either.

A fourth use of herbs or vitamins as antioxidants may seem innocent, but it is potentially damaging. Chemotherapy and radiation therapy work by creating free radicals that destroy cancer cells. Researchers warn that taking high levels of antioxidant supplements during these therapies could actually counteract their effectiveness.

Products without proven benefits may be safe for patients who want to try them. But you should always consult with your doctor in case there are particular risks for you.

Nutrition Notes is provided by the American Institute for Cancer Research in Washington, D.C.

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