Using Science to Dispel Herbal Safety Myths, Misconceptions

April 15, 2010

5 Min Read
Using Science to Dispel Herbal Safety Myths, Misconceptions


by Zoë Gardner


The majority of herbs used as dietary supplements have excellent safety profiles that have been confirmed by thousands of years of traditional use. Research on the safety and efficacy of botanicals has grown tremendously in the past 10 years (Figure 1), and generally confirms knowledge that has been documented in the traditional literature.

In spite of good research and documentation, misinformation on the safety of botanical supplements is common in the media, medical journals and numerous Web sites. Much of the information available has been incorrectly interpreted, lacks important details or is extrapolated from minimal data. Reading news headlines on the "dangers of herbs" or reviews in some medical journals would lead consumers and health care practitioners to conclude things such as:

  • Herb-drug interactions are widespread and common.

  • Kava and black cohosh have caused hundred of cases of liver damage.

  • Echinacea can affect heart rate.

A proper look at the scientific and traditional literature indicates none of these are true.

Current research suggests, in clinical practice, herb-drug interactions are relatively uncommon.1 Most of the popularly used herbs, including kava, milk thistle, Echinacea, black cohosh and valerian have been shown to not have any clinically relevant interactions.2,3,4 St. John's wort, however, is metabolized by an enzyme that metabolizes a number of prescription drugs, leading to changes in efficacy in drugs such as the immune-suppressant cyclosporin, some birth control pills and HIV protease inhibitors.5 Herbs with significant levels of mucilages (e.g., slippery elm or psyllium) may affect drug absorption and are best taken separately from drugs or other supplements.6

Analyses of published cases of kava-associated liver toxicity indicate out of more than 80 case reports, only one was agreed by multiple reviewers to be probable, and 12 more were thought to be likely related to kava.7,8 Comparing likely adverse reactions to total doses taken in Europe suggests the likelihood of an adverse reaction to kava is 1 in 4.1 million.9 Changes in liver enzyme levels, which would precede any liver toxicity, have not been observed in any clinical trials with kava or in average kava drinkers in nations where kava is consumed daily.10 A recent analysis of black cohosh products associated with liver toxicity found several products were not black cohosh but instead a closely related species.11 A review of nine published cases of liver toxicity associated with black cohosh indicated only one of the cases was possibly associated with this herb, and the other eight were not likely associated.12

A concern regarding effects of Echinacea on heart rate has been repeated in a number of articles. Tracing the literature back through several layers indicates these concerns come from unfiltered case reports and isolated incidents in clinical trials.13 A study looking specifically at effects of Echinacea on the cardiovascular system showed no concerns.14

While a review of the literature suggests the overwhelming majority of herbs used as dietary supplements have an excellent safety profile, a few herbs have effects that may be of concern to certain populations (e.g., pregnant or nursing women). Communicating herbal safety concerns openly and intelligently with consumers and health care providers helps to dispel myths, allay fears, and promote the safe and confident use of herbal supplements.


Zoë Gardner is program coordinator of the Medicinal Plant Program at the University of Massachusetts, which provides education, research and outreach related to all aspects of medicinal plants. She is the research editor of the American Herbal Product Association's (AHPA) forthcoming revision of the Botanical Safety Handbook, and is currently completing her Ph.D. in medicinal plant quality and safety.

References on the next page ...


References for Using Science to Dispel Herbal Safety Myths, Misconceptions

1. Sood A et al. Potential for interactions between dietary supplements and prescription medications. Am J Med. 2008;121(3):207-211.

2. Gurley BJ et al. In vivo effects of goldenseal, kava kava, black cohosh, and valerian on human cytochrome P450 1A2, 2D6, 2E1, and 3A4/5 phenotypes. Clin Pharmacol Ther. 2005;77(5):415-26.

3. Gurley BJ et al. Effect of goldenseal (Hydrastis canadensis) and kava kava (Piper methysticum) supplementation on digoxin pharmacokinetics in humans. Drug Metab Dispos. 2007;35(2):240-5.

4. Gurley BJ et al. Clinical assessment of CYP2D6-mediated herb-drug interactions in humans: Effects of milk thistle, black cohosh, goldenseal, kava kava, St. John's wort, and Echinacea. Molec Nutr Food Res. 2008;52(7):755.

5. Henderson L et al. St Johns wort (Hypericum perforatum): Drug interactions and clinical outcomes. Br J Clin Pharmacol. 2002;54(4):349-356.

6. Brinker F. Herb Contraindications and Drug Interactions. 2001. Eclectic Medical Publications, Sandy, OR.

7. Schmidt M. Is kava really hepatotoxic? 2007; p. 151. Accessed March 2010:

8. Schmidt M et al. Kava: A risk benefit assessment. In: The Essential Guide to Herbal Safety. 2005; p 115-203. Elsevier, St. Louis, MO.

9. Ibid

10. Schmidt M. Op cit.

11. Painter D, Perwaiz S, Murty M. Black cohosh products and liver toxicity: update. Canadian Adverse Reaction Newsletter, Health Canada. 2010;20(1):1-2.

12. Teschke R et al. Black cohosh hepatotoxicity: quantitative causality evaluation in nine suspected cases. Menopause. 2009;16(5):956-65.

13. Huntley AL, Coon JT, Ernst E. The safety of herbal medicinal products derived from Echinacea species: a systematic review. Drug Safety. 2005;28(5):387-400.

14. Shah SA et al. Effects of echinacea on electrocardiographic and blood pressure measurements. Am J Health Syst Pharm. 2007;64(15):1615-1618.

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