Dietary Supplement Interactions

A 1997 survey published in the Archives of Internal Medicine indicated that 18 percent of prescription drug users were also using herbal or vitamin and mineral dietary supplements. The authors estimated that this placed approximately 15 million people at risk of interactions between drugs and dietary supplements, 3 million of whom were 65 years and older.
The results of a 2005 survey published in the Journal of the American Dietetic Association indicated that as the use of non-vitamin, non-mineral supplements (such as herbal and botanical dietary supplements) had increased during the previous 10 years, so had the concomitant use of dietary supplements and over-the-counter and/or prescription drugs. Many of the people who participated in the survey were taking multivita-min pills and were surprised to learn that their multivitamins contained non-vitamin, non-mineral dietary (that is, herbal and other) supplements as well.

Interactions with Drugs

Few studies have specifically examined interactions between drugs and dietary supplements in the general population, and existing reports have found only mild interactions. These articles comprised mainly case reports, estimations based on laboratory experiments (not in humans), and speculation based on pharmacology studies and theoretical risks. Lack of data, however, does not imply lack of danger.
Dietary supplements may alter the activity of conventional drugs. Such interactions may occur in individuals who are particularly sensitive or immunocompromised or for drugs that have a narrow therapeutic window. In addition, response to drug and dietary supplement consumption may change with age and health status. Interactions between drugs and dietary supplements are especially likely in geriatric patients because they use more drugs and often combine prescription and over-the-counter drugs with herbal remedies. People with a chronic illness, specifically those with hepatic or renal impairment, are at higher risk of harmful interactions between drugs and dietary supplements.
Drug concentration or activity in the blood may be increased if a dietary supplement aids the absorption of the drug or if it inhibits enzyme destruction or elimination of the drug. Conversely, drug concentration and/or activity in the blood may be decreased if the dietary supplement binds components of the drug, thus preventing its absorption, or if the dietary supplement stimulates production and/or activity of enzymes that destroy the drug.
Some of the most popular dietary supplements may interact with drugs. A 2004 study at the University of Chicago Medical Center, which was funded by the National Institutes of Health and the Tang Center for Herbal Medicine Research, found that ginseng, which is commonly taken to enhance well-being, reduce fatigue, and improve immune response, decreased the effect of the anticoagulant warfarin. People who consumed warfarin and ginseng had an increased incidence of blood clots and therefore an increased risk of deep vein thrombosis. Because of ethical reasons, this study was done in healthy individuals, but the researchers believed that the results could be applied to those taking warfarin for prevention of deep vein thrombosis.
St. John's wort, which people commonly take for mild to moderate depression, may alter the effectiveness of drugs prescribed for HIV, heart disease, depression, epileptic seizures, and cancer and may interfere with oral contraceptives. Dietary supplements containing garlic have the potential to increase the risk of bleeding if combined with other drugs that increase bleeding risk, such as aspirin and warfarin. Specific interactions are listed in Table 3.4, but additional resources should be consulted to determine your risk of drug—dietary supplement interactions.

Interactions with Other Dietary Supplements

Although the potential for a dietary supplement to interact with others exists, currently there is not enough evidence to identify those risks. This should not be interpreted as there being no risk, however, only that none has been identified.