A bout 42% of Americans report taking medicinal herbs, and 16% say that they use herbal products simultaneously with prescription drugs.

Problem: Several of the top-selling herbal remedies interact with drugs and may cause dangerous -- sometimes even life-threatening -- side effects.*

What happens: Some herbs increase the effects of drugs by altering liver enzymes that break down the active molecules. Other herbs can decrease drug effects by competing for space in cellular receptors, molecular docking sites where drugs are active. Herbs also can increase or decrease drug absorption through the intestinal wall.

The potential risks are serious enough that some pharmaceutical companies have begun listing herb-drug interactions on their labels.

Scientists have found that the following herbs can cause dangerous interactions...

BLACK COHOSH

This herb often is taken to reduce hot flashes, night sweats and other menopausal symptoms.

Possible interactions: Black cohosh may increase the hormone-altering effects of many drugs, including birth-control pills, the cancer drug tamoxifen (Nolvadex) and the osteoporosis drug raloxifene (Evista). Other drugs that may interact with black cohosh include blood pressure drugs, medications that increase the risk for bleeding, such as warfarin (Coumadin), aspirin or nonsteroidal anti-inflammatory drugs, antiseizure drugs and antidepressants.

Bottom line: Check with your doctor and pharmacist before using black cohosh if you are taking any of the drugs listed above. Liver damage has been reported.

ECHINACEA

Many people take this herb to fight cold symptoms.

Possible interactions: Echinacea may alter immune-system activity. It also may interact with drugs metabolized via hepatic (liver) pathways, such as steroids and the rheumatoid arthritis drug methotrexate (Amethopterin). Other drugs that may interact with echinacea include antifungal drugs, acetaminophen (Tylenol) and antibiotics.

Bottom line: Check with your doctor and pharmacist before using echinacea if you are taking the above drugs.

GARLIC

Garlic supplements can lower both cholesterol and blood pressure levels by 5% to 10%.

Possible interaction: The antihypertensive effects of garlic supplements could cause blood pressure to drop too low in patients taking prescription blood pressure-lowering medication.

Other interactions: Some reports state that garlic supplements intensify the effects of anticoagulant medications, such as warfarin (Coumadin), and increase the effects of cholesterol-lowering statins. Thyroid drugs and supplements that have a blood-thinning effect, such as fish oil, also may interact with garlic.

Bottom line: Patients taking any of the drugs listed above should check with their doctor and pharmacist before using garlic supplements.

GINKGO BILOBA

Some evidence suggests that this herb boosts memory and mental alertness. It also may improve brain circulation and reduce symptoms of dementia due to stroke or Alzheimer's disease.

Possible interaction: Ginkgo increases the risk for bleeding when combined with warfarin and other drugs with blood-thinning effects, such as aspirin or ibuprofen.

Other interactions: Ginkgo may intensify the effects of antihypertensive drugs and lower both diastolic and systolic blood pressure levels. It also may increase insulin levels. Antidepressants, drugs that alter blood sugar levels, antiseizure drugs and supplements with blood-thinning effects also may interact with ginkgo.

Caution: Ginkgo should not be taken starting approximately two weeks before surgery. Check with your doctor and pharmacist.

Bottom line: Because the bleeding risk associated with taking ginkgo is well established, it should not be taken with the above drugs. Patients interested in improving mental function may want to try daily physical and mental exercise and stress-reduction techniques.

GINSENG

There's some evidence that ginseng improves concentration and reaction times. It may slightly reduce blood sugar (glucose) levels in patients with diabetes.

Possible interaction: Ginseng may alter the effects of warfarin. Other drugs that may interact with ginseng include medication that lowers blood sugar levels, digoxin (Lanoxin), blood pressure medication, over-the-counter medication for treating cold symptoms, such as pseudoephedrine (Sudafed), diuretics, central nervous system stimulants such as methylphenidate (Ritalin), corticosteroids, and antipsychotics.

Bottom line: If you take any of the drugs listed above, check with your doctor and pharmacist before using ginseng.

KAVA KAVA

This herb acts as a mild sedative and eases insomnia and anxiety.

Possible interaction: Kava kava may intensify the effects and prolong the action of other sedatives, especially alprazolam (Xanax), diazepam (Valium) and related benzodiazepines. Other drugs that may interact with kava kava include medications that affect the liver, such as methotrexate, and antifungal drugs, monoamine oxidase inhibitors (MAOIs), blood-thinners, such as warfarin, and other supplements with similar effects.

Bottom line: Avoid using kava kava with any of the drugs listed above.

Caution: In 2002, the FDA issued an advisory warning consumers that kava kava can cause liver damage.

Because study is still under way to determine which herbs are most likely to cause serious interactions with prescription drugs, always use caution and pay attention to your body's responses.

To read more about other herb-drug interactions, go to
www.naturalstandard.com, a Web site dedicated to the scientific study of integrative medicine.

*The FDA does not regulate herbal supplements. Consult your doctor and pharmacist before taking any herbal remedies.