Herb-Drug Interactions

Herb/Drug Interactions: Fact or Speculation?

One of the most common questions about herbal products these days is the potential interaction with prescription or over-the-counter (OTC) drugs. Surprisingly, little information is available about possible herb/drug or even herb/herb interactions. In fact, there is very little known about drug/drug or drug/food interactions. Our understanding of how the hundreds of substances we consume might affect one another is in its infancy.

For example, it was discovered that grapefruit juice affects the liver enzyme system that metabolizes certain drugs. The effect is strong enough that sensitive people can overdose on their prescription drugs if they are frequently taken in combination with grapefruit juice. Since that discovery, researchers have learned that substances in broccoli, cauliflower, wheat, corn, coffee, tea, and tobacco also affect this liver enzyme system.[i] Simply put, there are many other discoveries to be made in this emerging science.


Well then, what DO we know?

The first thing to keep in mind is that most of the fears surrounding herb/drug interactions are unfounded. They are mainly the result of exaggerated claims made by the media and of secondhand, misinterpreted information. In general, herbs are safe and do not interfere with the effects of conventional medications.1,[ii] However, while we wait for more accurate and rigorous research and better interpretations of herb/drug interactions, apply the following rules:2

  1. If you are taking any drug and wish to take herbs as well, it is best to seek the advice of a professional trained in herbal therapy. To find an experienced herbalist in your area, contact:

American Association of Naturopathic Physicians, tel. 1-866-538-2267, website: www.naturopathic.org

American Herbalist Guild (AHG); tel. (617) 520-4372 website: www.americanherbalistsguild.com

  1. The following drugs have a narrow therapeutic window (meaning that they can become dangerously toxic or dangerously ineffective with only relatively small changes in their blood concentra­tions): Digoxin, CoumadinÒ, anti-rejection drugs, anti-HIV drugs, Phenytoin, and Phenobarbital. Never take any herbal supplement with these drugs except under professional guidance.2
  2. If your heart, liver, or kidney function is impaired, if you are elderly, pregnant, if you have received an organ transplant, or if you have a genetic disorder that disturbs normal biochemical functions, do not take herbal supplements with drugs except under professional guidance.2
  3. Never take drugs and herbal supplements at the same time of day. Always separate them by at least an hour, preferably more.2
  4. If you have any type of serious disease and are being treated with prescription drugs, do not take any herbal supplement except under professional guidance.2
  5. Seek professional guidance before combining substances with similar effects. For example, taking synthetic sedatives along with alcohol or antihistamines is not recommended. For the same reason, using them with herbal sedatives may be ill advised.1
  6. Stop all herbal supplements about one week prior to surgery.2
  7. Research any herbal supplement you wish to take to see if there are known (not speculative) herb/drug interactions. Much is known, for example, about John’s wort, but there is still some speculation. Learn to differentiate between the two.
    • See the Informative Resources list below
    • Herb Research Foundation (HRF) at herbs.org

With a primary goal of public education, HRF is a nonprofit international education and research organization that collects and disseminates accurate and reliable information about natural health and herbs. You can also purchase extensive science-based papers on herbs and specific health conditions from their herb store at www.herbs.org/magento/index.php/

If you believe that an herbal supplement you are taking is causing an herb/drug interaction, stop taking it and seek professional advice.

It is valid to speculate about potential herb/drug interactions in order to define areas of caution. However, it is unwise to present speculations as confirmed fact, which is often found in the media. In reality, the best information about herb/drug interactions will come from case observations and scientific studies. In other words, they will be found by discovery, not speculation and extrapolation.2 Below is a list of some known herb/drug interactions.

Garlic and Ginkgo: Because of garlic and ginkgo’s blood-thinning effects, avoid taking concentrated garlic products in combination with prescription anticoagulant medications or other medications with blood-thinning effects, such as aspirin.1

Hawthorn: This herb may enhance the effects of digitalis (used for cardiac conditions), necessitating a lowered dose of digitalis.[iii] Blood pressure should be monitored in those taking beta-blockers.1

Kava: This herb should not be used with other substances that act on the central nervous system, such as alcohol, barbiturates, and medications for psychological disorders.[iv]

Licorice: Avoid taking glycyrrhizin or whole licorice root with thiazide diuretics, stimulant laxatives or other medications that contribute to potassium loss. Individuals taking licorice may be more sensitive to digitalis glycosides (used in cardiac conditions) and cortisol (used as an anti-inflammatory agent).4,5 Licorice products that include glycyrrhizin may increase blood pressure and cause water retention.[v] Some people are more sensitive to this effect than others. Long-term (more than two to three weeks) intake of products containing more than 1 gram of glycyrrhizin (the amount in approximately 10 grams of root) daily is the usual amount required to cause these effects. De-glycyrrhizinated licorice extracts, or DGL, do not cause these side effects because there is no glycyrrhizin in them.6,[vi]

Informative Resources:

Herb Contraindications and Drug Interactions: Plus Herbal Adjuncts with Medicine, 4th Ed. by Francis Brinker, N.D. (2010, Eclectic Medical Publications)

The Encyclopedia of Popular Herbs. Your Complete Guide to Learning Medicinal Plants by McCaleb, Robert. (2000, Prima Health)

A-Z Guide to Drug-Herb-Vitamin Interactions Revised and Expanded 2nd Edition: Improve Your Health and Avoid Side Effects When Using Common Medications and Natural Supplements Together by Alan R. Gaby (2006, Harmony)

Drug-Induced Nutrient Depletion Handbook by Ross Pelton, James B LaValle and Ernest B Hawkins (2001, Lexi-Comp)

Interactions Between Drugs and Natural Medicines. Chris Meletis, ND, Thad Jacobs, ND. Look for it at www.ncnm.edu/bookstore3/

Natural Medicines Comprehensive Database offers a consumer version that allows users to learn about herbs and other natural products, as well as check natural product/drug interactions and check nutrients depleted by common prescription and over-the-counter drugs. Available at: http://naturaldatabaseconsumer.therapeuticresearch.com

[i] McCaleb, Robert. The Encyclopedia of Popular Herbs. Your Complete Guide to Learning Medicinal Plants. Prima Health, Roseville, California. 2000.

[ii] Bone, Kerry, FNIMH. Herb/drug interactions: Real threats to your health or just exaggerated claims? Nutrition and Healing. Vol 8, Issue 3, March 2001.

[iii] McGuffin M, Hobbs, C, Uton R, et al, eds. American Herbal Products Association Botanical Safety Handbook. Boca Raton, FL. CRC Press, 1997.

[iv] Bluementhal M, Busse W, Goldberg, A, eds. The Complete German Commission E Monographs. Austin, TX: American botanical Council. Boston: Integrative Medical Communications, 1998.

[v] Blumenthal M, Busse WR, Goldberg A, et al. (eds). The Complete Commission E Monographs: Therapeutic Guide to Herbal Medicines. Boston, MA: Integrative Medicine Communications, 1998, 161–2.

[vi] Healthnotes. Online, Inc. 1505 SE Gideon St., Suite 200, Portland, OR 97202, www.healthnotes.com. 1999. Author are Lininger, Skye, D.C., Wright, Jonathan, M.D., Austin, Steve, N.D., Brown, Donald, N.D. & Gaby, Alan, M.D. Licorice.

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