What's the Story? Drug-Supplement Interaction

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At least 40% of all Americans take some type of dietary supplement. Although people usually take supplements in an effort to improve their health, some people may actually endanger their health by using these products-especially if they also take medicines. Some dietary supplements may interact in dangerous ways with prescription or over-the-counter medicines, and some may cause side effects even when taken alone.

Who is at risk?

Anyone who takes both dietary supplements and medicines may be at risk for drug-supplement interactions. However, the risks are greater for older people than for young adults. Physiological changes that accompany the aging process can increase the risk of side effects from both supplements and drugs. In addition, older people take more medications than younger people do ("polypharmacy"), and thus they are at increased risk of interactions. Although people over the age of 65 make up only about 13 percent of the total population, they use about 30 percent of all prescription drugs and at least 25 percent of all over-the-counter drugs. Two-thirds of adults over age 65 use one or more drugs each day, and one-fourth of the people in this age group take three or more drugs each day.

National health surveys have shown that people in their fifties and older are more likely than younger people to be supplement users. For example, in a recent survey conducted by the American Dietetic Association, daily supplement use was reported by 65 percent of respondents age 55 and over, as compared to 47 percent of those age 35 to 54 and 35 percent of those age 25 to 34. Since older people are also the heaviest users of medications, their high rate of supplement use may be a cause for concern.

In a survey commissioned by the American Council on Science and Health (ACSH), more than half of people over the age of 50 reported that they took prescription or over-the-counter medicines on a regular basis, and more than half of these people said that they also took dietary supplements. Thus, more than one-fourth of the 1825 older adults who participated in this survey were at risk for drug-supplement interactions.

What kinds of supplements interact with drugs?

Many of the drug-supplement interactions that have been reported in the medical literature have involved herbal supplements, such as kava, ginkgo biloba, licorice, and St. John's wort. However, the problem of drug-supplement interactions is not limited to herbs. Some vitamins and minerals-such as vitamin E, folic acid, calcium, and zinc, can interact with certain types of drugs. For example, vitamin E may interact with anticoagulants. Other types of non-herbal supplements, such as melatonin and S-adenosylmethionine, can also be involved in interactions.

How can drugs and supplements interact?

Drugs and supplements may interact in a variety of ways. In some instances, a drug and a supplement may have similar actions in the body. Taking both at the same time can cause an excessively strong effect. For example, combining an anticoagulant drug such as warfarin with a supplement that also inhibits blood coagulation, such as high-dose vitamin E, feverfew, fish oil, garlic, or ginkgo biloba, could increase the risk of abnormal bleeding.

In other instances, a supplement may counteract the effect of a drug. For example, supplements that tend to stimulate the immune system, such as vitamin E, zinc, or the herb echinacea, could interfere with the actions of drugs that are designed to suppress the immune system, such as corticosteroids or cyclosporine.

Some dietary supplements may affect the absorption of a drug from the digestive tract into the bloodstream. For example, if calcium supplements are taken at the same time as levothyroxine (a drug used to treat thyroid problems) or tetracyclines (antibiotics used to treat infections), less of the drug may be absorbed. As a result, the drug won't be as effective as it should be.

Other supplements may affect the way that the body breaks down a drug. For example, the herb St. John's wort increases the activity of an enzyme that breaks down a variety of drugs-including some drugs used in the treatment of heart disease, cancer, or AIDS as well as drugs used to prevent the rejection of organ transplants. If a patient who is taking one of these drugs also takes St. John's wort, the drug may be broken down more quickly than usual, and it won't be fully effective.

Are interactions a problem only for prescription drugs?

No. Dietary supplements may also interact with medicines that you can buy without a prescription, such as aspirin and cold remedies. In addition, some supplements may interact with alcohol. A few dietary supplements may also interfere with diagnostic tests. For example, the use of high-dose vitamine C supplements can interfere with tests for occult blood in the stool.

Besides drug interactions, are there any other risks associated with dietary supplements?

Yes. Some dietary supplements can have serious side effects even when taken alone. For example, supplements containing ephedra (also called ma huang) may damage the heart and nervous system and have been linked to at least 17 deaths. Several herbs, such as chaparral, comfrey, and germander, can cause potentially fatal liver damage. Some Asian herbal remedies contain components that can cause severe kidney damage.

Even vitamins and minerals can be harmful if taken in excessive doses. For example, overdoses of vitamin A can damage the liver. Taking too much vitamin D can damage the kidneys and cause calcium to be deposited in the soft tissues of the body. High doses of vitamin B6 can cause neurological damage, with symptoms such as numbness, weakness, and difficulty in walking. Overdoses of selenium can cause brittle nails, hair loss, and gastrointestinal and neurologic problems.

Some dietary supplements are safe for some people but not for others. For example, garlic and ginger seem to be reasonably safe for most people, but both of these herbs may be unsafe for diabetics. Taking too much magnesium may cause only a mild case of diarrhea in a healthy individual-but it can kill a person with kidney disease.

Consumers need to be aware that the quality of dietary supplement products may vary. Some products may not contain the amounts of ingredients stated on the label, or their composition may vary from batch to batch. Independent laboratories have begun testing dietary supplements and listing those that conform to their label statements. One source of such information that is readily available to consumers is the website <www.consumerlab.com>.

Do dietary supplements have to be shown to be safe and effective before they can be sold?

No. Dietary supplements are very loosely regulated in the United States. Unlike prescription and over-the-counter drugs, dietary supplements do not have to be proven safe or effective before they can be marketed.

The unusual regulatory status of dietary supplements is a result of the Dietary Supplements Health and Education Act (DSHEA), which was passed by Congress in 1994. This law created a new framework for the regulation of dietary supplements, gave supplement manufacturers the freedom to market a wider variety of products as dietary supplements, and allowed manufacturers to make claims about how their products affect the "structure and function" of the body. Dietary supplements are a unique regulatory category; they are not regulated in the same way as food additives or drugs.

Although the passage of DSHEA reflected the American public's strong desire for unimpeded access to dietary supplement products, it also placed a great burden on supplement users. Unlike users of other types of health care products, supplement users cannot be confident that the products they buy have met official standards for safety, efficacy, or quality. Supplement users have to educate themselves about the risks and potential benefits of the products that they use.

What can consumers do to minimize their risk of supplement side effects and drug-supplement interactions?

The most important thing that consumers should do is to tell their physicians about all dietary supplements that they are using or considering using. This precaution is important for everyone, but it is especially crucial for elderly people, pregnant women and those who may become pregnant, nursing mothers, people with chronic health problems, patients who will be undergoing diagnostic tests, and anyone who is taking any type of prescription or over-the-counter medication.

In addition, ACSH recommends that supplement users take these precautions:

  • Don't take supplements that provide more than the Tolerable Upper Intake Level (UL) of any vitamin or mineral unless your physician has specifically advised you to do so. If you don't know the UL (or if none has been established for a particular nutrient), don't take more than 100 percent of the Daily Value (DV).
  • Don't take supplements that contain any of these particularly risky ingredients: chaparral, comfrey, ephedra (ma huang), gamma butyrolactone, germander, lobelia, wormwood, or yohimbe.
  • Don't use dietary supplements as a substitute for medical treatment for any health problem. See your doctor first, have your problem diagnosed properly, and then ask your doctor whether self-treatment with dietary supplements is appropriate.

The Bottom Line-Some drugs and dietary supplements don't mix. To reduce the risk of harmful drug-supplement interactions and supplement side effects, anyone who is taking any type of prescription or over-the-counter medicine should consult with a physician before taking any dietary supplement.

Alcohol Beta-carotene, chamomile, gamma-butyrolactone (GBL)*, kava, valerian
Anesthetics Kava, St. John's wort**, valerian
Anticoagulants (blood thinners) such as warfarin (Coumadin®)***; Angelica root, arnica flower, anise, asafoetida, bogbean, borage seed oil, bromelain, capsicum, celery, chamomile, clove, coenzyme Q10, danshen, devil's claw, dong quai, fenugreek, feverfew, garlic, ginger, ginkgo biloba, gin seng, goldenseal, green tea (in large amounts), horse chestnut, licorice root, lovage root, meadowsweet, omega-3 fatty acids (fish oil), onion, papain, parsley, passionflower herb, poplar, quassia, red clover, rue, St. John's wort+, sweet clover, turmeric, willow bark, vitamin E
Antidepressants Ginkgo biloba, S-adenosylmethionine (SAMe), St. John's wort**, yohimbe*
Aspirin Willow bark; it is also possible that any of the drugs that interact with anticoagulants might also interact with aspirin
Diuretics Aloe, goldenseal, licorice
Drugs that lower blood cholesterol levels (such as the "statin" drugs) Red yeast rice
Drugs that suppress the immune system, such as corticosteroids or cyclosporine Echinacea, St. John's wort**, vitamin E, zinc; in addition, ginseng and melatonin may interact specifically with steroids
Drugs to treat diabetes Chromium picolinate, milk thistle
Drugs to treat heart problems Aloe, huperzine A, licorice, St. John's wort+
Drugs to treat high blood pressure Aloe, goldenseal, huperzine A, licorice, St. John's wort**
Drugs used to treat seizure disorders (such as phenytoin, phenobarbital) High-dose folic acid, ginkgo biloba, St. John's wort**, wormwood*
Estrogen replacement therapy (or combination estrogen/progestin hormone replacement therapy) Black cohosh, DHEA, saw palmetto
Methotrexate (a drug used in the treatment of cancer and sometimes also in the treatment of other diseases, such as rheumatoid arthritis and psoriasis) Echinacea, high-dose folic acid
Monoamine oxidase inhibitors**** Ephedra*, ginseng, melatonin, SAMe, St. John's wort**, yohimbe*
Phenothiazines (a group of drugs used to treat serious mental illnesses) Evening primrose oil
Phenylpropanolamine or pseudoephedrine (found in some cold remedies, allergy remedies, and diet pills) Ephedra*, yohimbe*
Protease inhibitor and nonnucleoside reverse transcriptase inhibitor drugs used in the treatment of HIV infection St. John's wort**
Tamoxifen (a drug used in the chemoprevention of breast cancer and also in cancer treatment) St. John's wort**
Tetracycline antibiotics Calcium, St. John's wort**
Thyroid hormone (Synthroid®) Calcium
Tranquilizers or sedatives Chamomile, guarana, kava, melatonin, St. John's wort**, valerian

* The supplements marked with an asterisk can cause very serious side effects, even when taken alone. ACSH recommends that no one should use them.

** Because the extent to which St. John's wort interacts with medications is only beginning to be understood, ACSH recommends that anyone who is taking any crucially important medication should not use this herb unless the physician who prescribed the drug has specifically approved its use.

*** More food and drug interactions have been reported for Coumadin than for any other medication. Patients taking Coumadin should not start or stop taking any other medications or alternative therapies or make any substantial changes in their eating habits without consulting their physicians.

**** Because there is a potential for extremely serious interactions between monoamine oxidase inhibitors and a variety of food components, dietary supplements, and other drugs, anyone who is taking this type of medication should scrupulously follow all of the dietary and other instructions provided by the physician who prescribed the drug.