More than two-thirds of Americans use complementary and alternative medicine (CAM) treatments, and dietary supplements are some of the most popular treatments used.¹ Some treatments can produce adverse events, including interactions with prescription medications. These authors designed a study to assess which are the most common clinically significant interactions among patients who concurrently use dietary supplements and prescription medications.

The study involved a cross-section, point-of-care survey of patients treated in six different specialty clinics (Departments of General Internal Medicine, Oncology, and Physical Medicine; the Fibromyalgia Clinic; the Preoperative Clinic; and the Spine Center) at the Mayo Clinic in Rochester, Minnesota. The authors designed an 85-question survey to determine the use of CAM therapies during the six months before the survey date. The questions asked about the use of 52 listed dietary supplements and included an opportunity for patients to document the use of supplements not included in the list. The survey was completed by 1,818 patients from September 2002 through July 2003.

Dietary supplements were defined as products containing herbs or other botanical components, amino acids, enzymes, organ tissues, glandular extract, or metabolites. (Vitamins and minerals were not included.) Prescription medications were defined as pharmacologic agents that were available in the United States only by prescription from a physician (though aspirin was also included).

The authors obtained information regarding the concurrent use of prescription medications from the patients’ electronic medical records. They acquired information on potential interactions by searching the MEDLINE database (using PubMed), the Natural Medicines Comprehensive Database, and the textbook Herb Contraindications and Drug Interactions: With Extensive Appendices Addressing Specific Conditions, Herb Effects, Critical Medications, and Nutritional Supplements. Clinical significance of individual interactions was assessed using the Lexi-Interact online medication and dietary supplement interaction analysis program.

The authors used the chi-squared and log-rank tests to compare the differences in baseline characteristics between patients who did and did not report use of dietary supplements. P values <0.05 were considered statistically significant. Statistical analysis was performed by using the SAS software package, version 9.1.3 (SAS Institute, Cary, NC).

The authors report an overall survey response rate of 98.7% (1,795 of the 1,818). Of the respondents, 710 patients (39.6%) reported use of dietary supplements. After excluding 11 patients because of a lack of information about the use of prescription medications in their medical records, 699 patients made up the final sample of dietary supplement users. Of these, the medical records of only 185 (26.5%) showed use of dietary supplements.

There were similar mean ages for those patients using dietary supplements (55.7 ± 14.9 years) and those not using supplements

(58.1 ± 16.8 years). Women were more likely to use supplements than men. Patients using supplements reported more symptoms than patients not taking the products (on average 10.6 ± 8.6 symptoms vs. 9.1 ± 7.7 symptoms, respectively). The mean self-perceived stress level ranked from 1 to 5 was statistically different between the two groups (P=0.008), but the actual difference of 0.1 was not clinically meaningful. The mean number of prescription medications per patient (3.6 ± 3.1 for supplement users vs. 3.7 ± 3.3 for nonusers) was not significantly different between the two groups (P=0.71).

The most common dietary supplements with a potential for interaction were preparations of garlic (Allium sativum, Alliaceae), valerian (Valeriana officinalis, Valerianaceae), kava (Piper methysticum, Piperaceae), ginkgo (Ginkgo biloba, Ginkgoaceae), St. John’s wort (Hypericum perforatum, Clusiaceae), glucosamine, ginger (Zingiber officinale, Zingiberaceae), and ginseng (Panax quinquefolius, Araliaceae). The most common prescription medication classes with a potential for interaction were antithrombotic medications, sedatives, antidepressant agents, and antidiabetic agents. The authors report that 369 potential interactions were identified among 236 patients. Of those, 107 interactions were considered to have potential clinical significance. The authors state, however, that no potential interactions resulted in serious harm (hospitalization, new medical problem, or serious bleeding) to any patient during the study.

Study strengths include the large sample size, excellent response rate, inclusion of patients from different specialty groups, and availability of complete data for most patients. Study weaknesses include the possibility of recall bias, possible incomplete or inaccurate medical record information about prescription medications, incomplete information about potential interactions between dietary supplements and prescription medications, and a limited generalizability of the study findings to the community.

The authors compared their study with other studies reported in the medical literature. Among all prescription medications, antithrombotic agents are the best-described class of medications with a potential for interaction with herbal products.^2,3 Findings from the study reported here confirmed this finding.

In summary, the present study showed that a small number of prescription medications and dietary supplements accounted for most potential interactions.

—Shari Henson