A Pharmacist's Guide to OTC Therapy: Herbal Supplements


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.

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

• Ginkgo may reduce the effectiveness of anticonvulsants

• Ginseng may intensify the effects of antihypergylcemic drugs, thus increasing incidence of hypoglycemia

• St. John's wort can cause a decrease in the plasma concentrations of agents such as amitriptyline cyclosporine, digoxin, and theophylline. In addition, St. John's wort also may cause breakthrough bleeding and decrease the effectiveness of oral contraceptives

• Valerian may produce an increased sedative effect when used with alcohol and other central nervous system depressants

• Kava may reduce the efficacy of levodopa and is contraindicated for use with other agents and herbs that may cause hepatic damage

• Garlic supplements may intensify the effects of some antihypertensive agents

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.


Most Commonly Used Herbal Supplements

Herbal Supplement

Examples of Common Uses

Potential Adverse Effects


Immune stimulant to prevent and treat colds and other upper respiratory infections

Fatigue, dizziness, headache, and GI symptoms


Hypercholesterolemia, hypertension, peripheral arterial disease

Nausea; burning sensation in mouth; throat, and stomach; halitosis; and body odor

Ginko biloba

Cognitive impairment, vascular dementia, tinnitus

Nausea, dyspepsia, headache, and heart palpitations

Saw palmetto


Headache and diarrhea


Mental and physical stress, anemia, diabetes, and insomnia

Anorexia, rash, changes in blood pressure, and headache

Grape seed extract

Chronic venous insufficiency, diabetic retinopathy, and atherosclerosis

No adverse effects reported in literature

Green tea

Protections against cancer, cardiovascular disease, and hepatic disorders

Insomnia, anxiety, and tachycardia

St. John's wort

Depression, pain, anxiety, and insomnia

Photosensitivity, xerostomia, dizziness, and confusion


Vision impairment

No adverse effects reported in literature


Aloe Topical aloe: promotion of wound healing and dermatitis

Oral aloe: digestive disorders wash stimulation

Allergic reactions and urticaria. The use of topical aloe is contraindicated in individuals with allergy to plants in the Liliaceae family.

GI = gastrointestinal; BPH = benign prostatic hypertrophy.

Adapted from references 1, 5, and 6.


  1. Bent S, Ko R. Commonly used herbal medicines in the United States: a review. Am J Med. 2004;116(7):478-485.
  2. Sood A, Sood R, Brinker FJ, Mann R, Loehrer LL, Wahner-Roedler DL. Potential for interactions between dietary supplements and prescription medications. Am J Med. 2008;121(3):207-211.
  3. McQueen C, Hume A. Introduction to Botanical and Nonbotanical Medicines. In: Berardi R, Kroon L, Newton G, et al, eds. Handbook of Nonprescription Drugs.15th Edition. Washington, DC: American Pharmacists Association; 2006:1095.
  4. Hu Z, Yang X, Ho PC, et al. Herb-drug interactions: a literature review. Drugs. 2005;65(9):1239-1282.
  5. Hume, Anne and Strong, Kathryn. Botanical Medicines. In: Berardi R, Kroon L, Newton G, et al, eds. Handbook of Nonprescription Drugs.15th Edition. Washington, DC: American Pharmacists Association; 2006:1104-1136.
  6. Dermatological Disorders. In: Beers M, ed: The Merck Manual of Diagnosis and Therapy.18th Edition. Rahway, NJ: Merck Publishing; 2006:2724-2728.
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