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HOLISTIC HEALTH - February 2020

Common Herbal Dietary Supplement Drug Interactions


by Nancy Russell, M.D


A well known medical journal for physicians, “American Family Physician” advises physicians to discuss use of dietary supplements with their patients in an article, “Common Herbal Dietary Supplement-Drug Interactions”.

Estimates show that between 40-60 percent of U.S. adults with chronic disease use dietary supplements, and among those people taking prescription medications, an estimated 20- 25 percent concurrently use a dietary supplement. Accordingly, there has been increased concern for the potential for dietary supplements, particularly herbs to interact with prescription medications. These interactions are infrequent, but can have consequences.

There are two ways that herbs can interact with prescription medications: pharmacokinetic versus pharmacodynamic. A pharmacokinetic reaction affects the drug’s concentration in the blood and its action on the body, which can be significant. In many cases, these pharmacokinetic reactions can be safely countered by adjusting the dosage of the prescription. In this type of interaction, the herbal supplement shares the same mechanism of absorption, distribution, metabolism or elimination of the prescription drug. Less commonly, herb-drug interactions manifest as pharmacodynamic interactions, when an herbal supplement has a direct effect on the mechanism of action of the prescription drug. This type of action makes the herb potentially harmful with the drug and should not be used. 

The article summarizes several commonly used herbal dietary supplements for interaction risks based on national surveys, research studies and clinical experience. Herbal supplements with overall low risk of drug interactions include black cohosh, cranberry, gingko biloba, american ginseng, milk thistle, saw palmetto and valerian. There are some exceptions which are mentioned in this article as seen below. 


  • Black cohosh does have some concerns for interactions with OATP2B1, which could reduce the effectiveness of such drugs as amiodarone, fexofenadine (Allegra), glyburide, and many of the statin medications. It should not be combined with these prescription drugs.

  • Ginkgo, also known as ginkgo biloba is known to inhibit platelet activity so theoretically could increase the risk of bleeding in a person. Caution should be given with people on the blood thinner warfarin (Coumadin) and the blood clotting test, INR should be monitored closely or refrain from using ginkgo biloba. American ginseng has a similar warning and should not be taken with Coumadin or the INR closely monitored. 

  • Milk thistle, also known as silybum marianum, depending on the person’s CYP2C9 genotype, may reduce the metabolism of losartan (Cozaar). There is also potential for milk thistle to decrease concentrations of other medications metabolized by CYP2C9 such as warfarin (Coumadin), phenytion (Dilantin) and diazepam (Valium). 

  • Asian ginseng (Panax Ginseng), has been shown to induce CYP3A4, which could decrease the effectiveness of many drugs, including calcium channel blockers, many chemotherapy and HIV agents, cholesterol lowering statin drugs and some antidepressants. For this reason, it is suggested to avoid use of Asian ginseng, including products containing Chinese, Japanese, and Korean ginseng, with most medications. Definitely avoid using these herbs with warfarin (Coumadin).

  • Curcumin has been shown to induce CYP1A2, which could cause a decrease in many antidepressants and antipsychotic agents. If has also been shown to increase sulfasalazine (Azulfidine) levels. 

  • Echinacea has been shown to effect CYP1A2 and CYP3A4 enzymes which effects the metabolism of medications that are metabolized by either enzyme. This would include many antipsychotic and antidepressant medications. Echinacea, since it boosts the immune system should not be given to a patient with auto-immune disease such as lupus, rheumatoid arthritis, scleroderma, etc.

  • Garlic should also be avoided by people with auto immune disease as it can also rev up the immune system. Garlic enhances the activity of white blood cells, particularly macrophages and lymphocytes. Garlic extract has been shown in human studies to decrease concentrations of drugs that are transported by P-gp, but it has no effect of CYP1A2, CYP2D6 or CYP3A4. Medications that are transported by P-gp include colchicine, digoxin, doxorubicin, quinidine, rosuvastin (Crestor), tacrolimus (Prograf) and verapamil. Each of those medications should not be combined with garlic supplements. 

  • Goldenseal herb has been shown to inhibit two major metabolic enzymes, which are responsible for metabolism of more than 50percent of currently used prescription medications. Although some drug combinations with goldenseal may be safe, until data from further human clinical trials are available, clinicians recommend against the use of goldenseal in combination with most other medications. 

  • Green tea extract herb has been investigated for potential drug interactions in humans and other studies with conflicting results. Green tea extract has been shown to increase simvastatin (Zocor) concentrations. Two additional studies have demonstrated that green tea extract may inhibit drug transport mechanisms and interferes with the action of many drugs, including statins, fluoroquinolones (cipro, levaquin), some beta blockers, imatinib (Gleevec), and antiretroviarls (used to treat HIV). Therefore, green tea extract should be avoided in combination with drugs that are transported by P-gp, OATP1A1, or OATP1A2. 

  • St. John’s Wort has been shown in multiple human studies to be a potent inducer of CYP3A4 and P-gp. Clinical studies have shown reductions in action of the following drugs; cyclosporine, tacrolimus, warfarin, protease inhibitors, irinotecan, theophylline, digoxin, venlafaxine, and oral contraceptives. It is strongly recommended to avoid concurrent use of St. John’s wort with most over-the-counter and prescription medications.


It is important for you and your medical provider to educate yourself on potential interactions between prescription medications and supplements. Human herb-drug interaction studies are being published regularly. Even though there are interactions known, overall these interactions are infrequent. Examples of reliable sources to investigate these interactions include PubMed, Natural Medicines Database, the Allied and Complementary Medicine Database, Lexi-Natural products, and the National Institutes of Health’s Office of Dietary Supplements. Be aware and be proactive to your health care decisions.  




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Nancy Russell, M.D. has been a holistic Internal Medicine physician in the Kansas City northland for over 30 years at 5140 N. Antioch Road in Kansas City, MO.


Her phone number is 816-453-5545 and website is where you can get more information. Dr. Russell is board certified in holistic medicine and is a member of the American Holistic Medical Association and a prior board member.

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