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The use of herbal supplements has increased in the last 10 to 15 years, and an estimated 25% of adults use one or more herbal supplements to treat a medical condition.1 Herbal supplements are defined as dietary supplements derived from a plant source, including the leaves, stems, flowers, roots, and seeds.1 Herbal supplements are available as single-entity or combination products. In addition, some multivitamin/mineral supplements are now formulated with herbal supplements, such as ginkgo biloba and ginseng.
According to an article by Bent and Ko, published in the April 2004 issue of the American Journal of Medicine, of the 10 most frequently used herbal supplements in the United States in 2001, only 4 herbs—garlic, ginkgo biloba, saw palmetto, and St. John's wort—have systematic reviews that report statistically significant evidence of efficacy.1
Many patients may assume that herbal supplements are generally safe because they are derived from natural sources; however, many patients with preexisting medical conditions and/or those patients concurrently taking other medications—including prescription and OTC agents—may not be awaretematic reviews that report statistically significant evidence of effi of the potential for drug–herbal supplement interactions.
Examples of Possible Herbal Supplement–Drug Interactions
• Supplements such as garlic, ginkgo, ginseng, St. John's wort, and evening primrose oil may increase the international normalized ratio in individuals taking warfarin, thus increasing the incidence of bleeding
Adapted from references 1, 2, 5, and 6.
Sood et al reported that the potential for significant drug–herbal supplement interactions appears to be most common in the following drug classes: antithrombotic medications, sedatives, antidepressant agents, and antidiabetic agents. These accounted for an estimated 94% of the potential clinically significant interactions.2 The drug most commonly documented for a potential drug–herbal supplement interaction is warfarin.2 Furthermore, Sood et al reported that the 5 most common herbal supplements associated with potential drug interactions are garlic, valerian, kava, ginkgo, and St. John's wort.2
Results from a national survey reveal that an estimated 18.4% of patients using prescription medications also used herbal remedies.2 In addition, many patients do not report the use of these herbal supplements, making it more difficult to screen for possible drug–supplement interactions or contraindications. Results from another survey reported that an estimated 63% of participants did not inform their primary health care provider about their use of dietary supplements.3 Sood et al concluded that, although the potential for interactions between prescription drugs and herbal supplements appears to be high, the actual potential for harm is relatively low.2
Pharmacists can be a fundamental source of information for patients seeking guidance about the safety and efficacy of herbal supplements. When counseling patients about prescription and OTC medications, pharmacists should ascertain if the patient is currently using herbal supplements in order to assess for possible drug interactions or contraindications. Likewise, when assisting patients in the selection of herbal supplements, pharmacists should screen for possible drug interactions and contraindications.
Factors to be considered when evaluating the clinical significance of an herbal supplement–drug interaction include the particular herb, the drug, and the medical history of the patient.4 Patients should be reminded to discuss the use of herbal supplements with their primary health care provider prior to using them and to always include these supplements as part of their medication profile. Women who are pregnant or lactating should never use an herbal supplement without consulting their primary health care provider.
In addition, it is important to remind patients to always use herbal supplements from reputable manufacturers and to adhere to the patient instructions provided. Patients also should be reminded that if they experience any adverse effects to report them to their primary health care provider immediately.
For more information regarding herbal supplements, please visit the National Institutes of Health's National Center for Complementary and Alternative Medicine at nccam.nih.gov.