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Top Herbal Products: Efficacy and Safety Concerns

  • Authors: Darrell T. Hulisz, PharmD
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Target Audience and Goal Statement

This activity is intended for pharmacists, primary care physicians, nurse practitioners, physician assistants, nurses, and all clinicians caring for patients who are likely to be using herbal remedies.

The goal of this activity is to review traditional uses, evidence-based data, mechanisms of action, side effects, contraindications and herb-drug interactions of the most popular herbal products on the US market.

Upon completion of this activity, participants will be able to:

  1. Describe traditional and evidence-based uses of the most commonly used herbal products
  2. Explain the probable mechanisms of action of these therapies
  3. Discuss side effects, contraindications, and drug interactions of the most commonly used herbal products


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Medscape, LLC encourages Authors to identify investigational products or off-label uses of products regulated by the US Food and Drug Administration, at first mention and where appropriate in the content.


  • Darrell T. Hulisz, PharmD

    Associate Professor, Department of Family Medicine, Case Western Reserve University School of Medicine, University Hospitals, Case Medical Center, Cleveland, Ohio


    Disclosure: Darrell T. Hulisz, PharmD, has disclosed that he has received grants for educational activities from Pfizer, Takeda, and Novartis. Dr. Hulisz has also disclosed that he has served as an advisor or consultant for Pfizer, Takeda, and Novartis.


  • Christine Wiebe, MA

    Editorial Director, Medscape Medical Students and Pharmacists


    Disclosure: Christine Wiebe has disclosed no relevant financial relationships.

  • Jacqueline A. Hart, MD

    Freelance Clinical Editor, Medscape, LLC, Boston, Massachusetts


    Disclosure: Jacqueline A. Hart, MD, has disclosed no relevant financial relationships.

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Top Herbal Products: Efficacy and Safety Concerns

Authors: Darrell T. Hulisz, PharmDFaculty and Disclosures



A recent survey estimated that more than 38 million adults in the United States used herbal products and dietary supplements in 2002.[1] More than half of those users said that these products were important to their health and well-being, yet only one third told a conventional healthcare provider about their use.

Other reports have estimated that 25% of patients who seek medical attention for a serious medical problem are using "unconventional" therapies, and 70% of those patients do not disclose those practices to their physician.[2,3] One survey estimated that about 18% of the US population uses herbal therapy on a regular basis.[4]

Herbal products and dietary supplements are widely available in supermarkets and other retail outlets, as well as by mail order. In fact, only a small percentage of these products (4.5%) are actually sold in pharmacies.[5] Many patients believe they derive health benefits from these herbal preparations, yet some remain skeptical and may seek advice from health professionals prior to use. Thus, it is important for all health professionals to be informed about available products and to be aware of any potential problems associated with their use.

Concerns About Herbal Products and Dietary Supplements

Unlike prescription and over-the-counter drugs, herbal products are not regulated by the US Food and Drug Administration (FDA) to determine purity or potency.[6] In fact, some products may contain contaminants, and their potency is dependent on many factors, such as the climate and soil conditions where they are grown, harvested, and stored.[7] Some herbal preparations have even been found to contain prescription drugs and heavy metals as unlabeled ingredients, and in some cases, these contaminants have resulted in toxicities.[8-17]

The labeling of herbal products is regulated by the Dietary Supplement Health and Education Act of 1994 (DSHEA).[18] Under this law, manufacturers may only make general claims about a supplement's effect on the structure or function of the human body (eg, "supports the immune system"). They must include the following statement in their labeling: "This product is not intended to diagnose, treat, cure, or prevent any disease."

The labels on herbal products are designed to promote product use and not necessarily to inform the consumer, so health professionals should be equipped with a general understanding of popular herbs and supplements, including knowledge of efficacy, common side effects, risks, and interactions. In addition, they should prospectively seek information regarding their patients' use of unconventional medicines to avoid adverse consequences.

Consumers should be advised that manufacturers of herbal supplements are not required to demonstrate safety or efficacy prior to marketing. Before the FDA can remove a product from the market, the agency must prove that the product is unsafe or ineffective. Recent examples of this include the ban on products containing ephedra due to adverse cardiovascular effects,[19] and the prohibition of kava amid concerns about hepatotoxicity.[20]

Patients with medical illness should not use herbs and dietary supplements without medical supervision. As will be discussed, some herbal products have adverse effects and may interact with prescribed medications. Furthermore, many conditions that patients try to diagnose or treat themselves may be serious, requiring a careful history and examination by a healthcare professional. For example, unsupervised use of saw palmetto for urinary symptoms may delay a diagnosis of prostate cancer.[21] Similarly, patients with symptoms such as chronic insomnia, anxiety, and depressed mood should see their health provider. Patients with cardiovascular disease, hypertension, heart failure, and hyperlipidemia should be under a healthcare professional's care and receive appropriate prescription drugs.

The following is an overview of some of the most commonly used herbal products, including important clinical considerations in the use of these products.

Echinacea for Fighting Cold Symptoms, Boosting Immunity

Echinacea is one of the most popular herbs in the United States, extracted from the purple coneflower that is native to North America. Species include Echinacea purpurea, Echinacea angustifolia, and Echinacea pallida. It has been studied as an adjunct therapy to enhance the immune system, mostly in upper respiratory tract infections, and these studies have produced mixed results.[22,23]

A recent meta-analysis concluded that standardized extracts of echinacea were effective in the prevention of common cold symptoms after clinical inoculation, when compared with placebo.[23] The authors of that study concluded that the likelihood of experiencing a cold was 55% higher with placebo than with echinacea (P < .043).

Some antiviral and bacteriostatic properties have been demonstrated in vitro, and the herb also appears to stimulate the production of cytokines (interferon, tumor necrosis factor, and interleukins).[24] Animal studies have revealed probable mechanisms of echinacea-induced immune enhancement, such as increasing the number of circulating white blood cells.[25]

Common side effects of echinacea supplements include unpleasant taste and allergic reactions. Because the flower is related to ragweed, cross allergenicity may occur in individuals allergic to ragweed.[26,27]

Echinacea is not recommended in patients with progressive or autoimmune disorders, including acquired immunodeficiency syndrome, tuberculosis, multiple sclerosis, collagen disorders, and diabetes mellitus. Theoretically, since echinacea alters the immune system, these disorders may be exacerbated.[28,29] Persistent use of echinacea has been associated with hepatotoxicity, so it should not be taken by patients who are taking other known hepatotoxins such as anabolic steroids, amiodarone, methotrexate, or ketoconazole.[30]

While not all randomized controlled trials performed to date have shown benefit for Echinacea over placebo,[22,31] a Cochrane review in 2006 concluded that the aerial parts of Echinacea purpurea might be effective for early treatment of colds in adults.[32]

Unfortunately, the optimal dose of echinacea is unknown, and multiple formulations exist, such as capsules, tinctures, teas, and extracted plant juice. Recommended dosages vary widely; for instance, capsules of Echinacea purpurea extract range from 100 to 500 mg, with manufacturers' suggested use ranging from 1 to 4 times daily for common cold prevention.

Take-home message: Clinicians should inform patients that a wide variety of Echinacea preparations and doses have been studied and results are inconsistent, making it difficult to recommend specific products. However, E. purpurea seems to be modestly effective for preventing the common cold in those at risk (eg, sick contacts). Evidence that the herb may reduce the duration of cold symptoms has been mixed. Patients allergic to ragweed, with progressive autoimmune disorders, and on hepatotoxic drugs should avoid echinacea.

Garlic: Warding Off Cardiovascular Disease?

Allium sativum, commonly known as garlic, has been used for centuries in cooking because of its flavoring properties. Today, it is used therapeutically by many consumers to prevent heart disease by controlling high cholesterol and high blood pressure. The suggested active ingredients are allicin and alliin. Numerous studies, however, have produced conflicting results regarding garlic's ability to lower lipids.[33-40] Positive findings in 3 trials exhibited a lowering of cholesterol in the range of 6.1% to 11.5%, primarily due to the lowering of low-density lipoproteins.[33-36] Other studies have yielded neutral or conflicting results.[37-39] One 12-week study tested the use of garlic powder in ambulatory patients, finding a 14% reduction of serum cholesterol.[40]

Garlic may have modest antihypertensive effects. Studies have documented either a small decline in arterial pressure (5% to 7% mm Hg) or no change at all.[41] A meta-analysis of 8 trials revealed 3 studies that concluded garlic significantly reduced systolic blood pressure, and 4 studies that found reductions in diastolic blood pressure in patients with mild hypertension.[42]

With regard to potential adverse effects, garlic has been shown to inhibit platelet aggregation in vitro[43-45] and in vivo[46]; thus, it should be used with great caution in individuals with bleeding disorders or in those who are receiving antiplatelet therapy.[47] There has been 1 report of a spontaneous epidural hematoma occurring with garlic supplementation, but this appears to be an isolated case.[48]

Garlic may also decrease warfarin concentrations.[49-51] Studies to date have not confirmed this interaction[52,53]; but particularly close monitoring of the international normalization ratio (INR) for patients taking both garlic and warfarin is prudent. Patients taking garlic supplements should discontinue use 7 to 10 days prior to having surgical procedures to avoid the potential for prolonged bleeding.[54]

The most prominent side effect of garlic supplementation is malodorous breath and garlic-like body odor.

Take-home message: Garlic should be used cautiously in individuals receiving antihypertensive medications, and blood pressure should be monitored carefully; orthostatic hypotension is a rare complication for those on antihypertensives.[55] Also, garlic should be avoided in those with a history of orthostasis or unexplained dizziness as well as in patients taking drugs that can increase bleeding, such as aspirin, warfarin and ibuprofen. A lack of standardization of garlic products and formulations makes it difficult to recommend a dose or specific product. For dyslipidemia, patients may benefit from taking 600 to 1200 mg of garlic powder daily in divided doses, or up to 4 g of raw garlic daily.

Ginkgo Biloba for Enhancing Memory, Combating Alzheimer's Disease

Ginkgo biloba is one of the oldest species of living trees, and ginkgo supplements are derived from the tree's leaves. This herbal remedy is marketed to improve memory, particularly in elderly individuals. While its mechanism of action is not fully understood, ginkgo contains flavonoids, terpenoids, and organic acids that are believed to act as free radical scavengers. These constituents have been shown to:

  • Inhibit platelet activation factor (reducing thrombosis);

  • Dilate arteries and capillaries; and

  • Block the release of chemotactic factors and inflammatory mediators.

Studies in the United States have found that ginkgo stabilized -- and in some cases improved -- cognitive function and socialization in patients with Alzheimer's disease, although the clinical significance of the improvement was not known.[56,57] LeBars and colleagues[56] reviewed 2020 patients in an intention-to-treat analysis that resulted in a 1.4-point advantage over placebo in the Alzheimer's Disease Assessment Scale-Cognitive subscale.

In addition, studies have also demonstrated that the standardized extract of ginkgo biloba (EgB) 761, is effective in reducing symptoms of claudication, giving patients a 50% increase in pain-free walking distance.[58]

In contrast, a recent clinical trial failed to demonstrate any improvement in cognitive function or in the quality of life in cognitively intact, older individuals.[59]

Ginkgo is considered relatively safe, although the leaves have been associated with mild gastrointestinal side effects and headache. Ingestion of ginkgo seeds may result in serious neurologic and allergic reactions; therefore, they are not used for medical purposes.[60,61] Ingestion of leaf-based extracts has been associated with a spontaneous hyphema (blood in the anterior chamber of the eye) in an elderly man,[62] and with spontaneous subdural hematomas.[63] However, while some isolated adverse events such as these exist, it is considered safe when used as directed.

Ginkgolide B, an active component of ginkgo biloba, is a potent inhibitor of platelet-activating factor, which is necessary for normal platelet aggregation. As with garlic, it should be avoided in patients using anticoagulants or antiplatelet therapy, or in those who have active bleeding such as peptic ulcer disease. Based on case reports, ginkgo is not recommended in patients with seizure disorders.[64-67]

Standardization of product and recommended dosing is lacking, but a typical dose is 40 mg to 80 mg taken 3 times daily, standardized to 24% to 27% ginkgo flavone glycosides and 6% to 7% triterpines per dose. A dose of ginkgo extract EGb 761 at 160 mg daily has shown equivalent efficacy compared with donepezil 5 mg daily for the treatment of Alzheimer's disease.[68]

Take-home message: Ginkgo is a reasonable therapeutic option in patients with Alzheimer's disease who are also receiving medical care, but providers should remember that the herb has antiplatelet activity and thus may not be appropriate for patients with bleeding disorder or on antiplatelet or anticoagulation agents.

St. John's Wort Used for Depression

This yellow flowering plant (Hypericum perforatum) is named after St. John the Baptist. Extracts of the flower have been used for centuries to treat mental illnesses. The herbal product has 10 constituents, of which hypericin is believed to be the most active ingredient in treating depression. St. John's wort has a high affinity for gamma-aminobutyric acid, which, when stimulated, produces an antidepressant effect.[69] Hypericin also appears to activate dopamine receptors and inhibit serotonin receptor expression. In vitro, it has been shown to block reuptake of serotonin and norepinephrine.[70] These mechanisms may explain the lag time associated with the effectiveness of the herb.

A number of studies have examined the effectiveness of St. John's wort in depression.[71,72] A meta-analysis of 23 controlled trials concluded that it was more effective than placebo in treating mild-to-moderate depression.[72] In a 12-week study of 135 depressed patients, the herbal extract (900 mg per day) was found to be more effective than fluoxetine (20 mg per day).[73] Other investigators have confirmed the herb's efficacy over placebo in mild-to-moderate depression.[74]

Because of its pharmacology, St. John's wort should not be taken with prescription serotonin uptake inhibitors, as symptoms of serotonin syndrome have been observed with co-administration (headache, sweating, dizziness, and agitation).[75-77, 85]

In terms of other contraindications, St. John's wort should be avoided during pregnancy.[78,79] It has been associated with photosensitivity.[80] Studies have shown that the herbal product can reduce plasma concentrations of digoxin[77,81] and indinavir.[77,82,83] There have been cases of heart transplant rejection associated with the use of St. John's wort that resulted from a reduction in cyclosporine plasma concentrations.[84,85] Breakthrough bleeding and unwanted pregnancies have been reported in women on concomitant therapy of oral contraceptives and St. John's wort extract.[85,86] This is likely a result of St John's wort-induced decreases in ethinyl estradiol or metabolite concentrations due to the herb's ability to induce cytochrome P450 (CYP450)-3A4 isoenzyme.

Take-home message: Patients who are depressed should not take this herb without medical supervision. St. John's wort should be reserved for the mildly depressed patient with an aversion to prescription medication. Clinicians who recommend this botanical should be mindful of the numerous potential drug interactions arising from enzyme induction. The most commonly studied dose for depression is 300 mg taken 3 times a day, standardized to 0.3% to 0.5% hypericin per dose.