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News Organizations Warn about the Potential for Herb-Drug Interactions

CBS News and other media outlets uncritically report the results of a flawed study that attempts to evaluate causality in alleged herb-drug interactions


On January 24, 2018, CBS News released a three-minute video segment titled “Why herbal supplements taken with prescription drugs may be risky,” based on a recent study that analyzed reports of serious herb-drug interactions (HDIs) from published clinical trials.1

In the “CBS This Morning” segment, medical correspondent Tara Narula, MD, an assistant professor of cardiovascular medicine at the Zucker School of Medicine in New York, explained that “herbal supplements are pharmacologically active” and that they “could interact with medication.”1 Cranberry (Vaccinium macrocarpon, Ericaceae), ginkgo (Ginkgo biloba, Ginkgoaceae), ginseng (Panax spp., Araliaceae), green tea (Camellia sinensis, Theaceae), and St. John’s wort (Hypericum perforatum, Hypericaceae) were singled out as botanical ingredients that can cause HDIs; however, the report failed to mention any specifics about the type of conventional drugs with which these botanical ingredients might interact.

While CBS News did not cite the source of its information, other recent media coverage on the topic, such as online reports from Science Daily2 and TIME,3 refers to a study by researchers from the South African Medical Research Council and the University of Stellenbosch published in the British Journal of Clinical Pharmacology (BJCP).4

The study assessed 49 case reports and two observational studies out of 5,113 retrieved articles that were published between 2001 and 2017. Using various assessment methods, the reported adverse drug reactions were classified according to the probability that HDIs were the cause. Based on their analysis, the authors ranked the likelihood of HDIs causing the reactions as “highly probable” (8%), “probable” (51%), “possible” (37%), or “doubtful” (4%).

The four “highly probable” case reports include alleged interactions between noni (Morinda citrifolia, Rubiaceae) juice and phenytoin (an antiseizure drug), cranberry juice and warfarin (a blood-thinning drug), goji (Lycium barbarum, Solanaceae) berry and warfarin, and St. John’s wort and cyclosporine (an immunosuppressant drug).

Ginkgo, ginseng, St. John’s wort, and goji each were involved in three case studies. Patients were being treated for a variety of conditions, including heart disease, cancer, kidney transplants, depression, schizophrenia, anxiety disorders, and seizures. These patients were taking conventional pharmaceutical drugs (e.g., warfarin, statins, chemotherapeutics, antidepressants, or immunosuppressants) and a majority had multiple conventional drugs prescribed.

The potential for clinically relevant HDIs is a significant concern for many health care practitioners. Since sufficiently rigorous clinical studies on the issue are lacking, in vitro research, a small number of case reports, and even hypothetical and theoretical concerns often are used as data sources to identify potential HDIs. The data from in vitro studies, particularly those on isolated liver enzymes, have been shown to be of little clinical relevance,5-7 and relying on such data may deprive a patient of using a beneficial herbal supplement.

Unfortunately, most media reports on the BJCP paper did not identify the reported interactions that were clinically relevant, but rather suggested that herbal ingredients, in general, may put one’s well-being at risk when taken with conventional drugs. As such, the herb-drug information reported in the mainstream media based on this new paper is inadequate.

Bill Gurley, PhD, a professor of pharmaceutical sciences at the University of Arkansas for Medical Sciences who has researched clinically relevant HDIs for more than 20 years, commented: “In my opinion, this current media reporting is much ado about nothing, especially given the confounding variables in most of the case reports. Case reports are fraught with unknowns, and supplement dosage forms are also oftentimes fraught with unknowns.”

Gurley continued: “As far as the [BJCP] paper goes, it’s really nothing we didn’t already know, except that several of the case reports they cited for botanicals like goji, ginseng, and ginkgo are simply too speculative to give them much clinical merit. There are some botanicals that do pose a significant risk for drug interactions, but goji, ginseng, and ginkgo are not among them. At least, they do not pose a risk when taken responsibly.”

The following eight herbs and common household foods, depending on levels of intake, are known to produce potential clinically relevant HDIs of which both consumers and health care practitioners should be aware: black pepper (Piper nigrum, Piperaceae), goldenseal (Hydrastis canadensis, Ranunculaceae) root, grapefruit (Citrus paradisi, Rutaceae), green tea, licorice (Glycyrrhiza spp., Fabaceae), milk thistle (Silybum marianum, Asteraceae), schisandra (Schisandra chinensis, Schisandraceae), and St. John’s wort.

Consumers who are using pharmaceutical drugs and are considering taking any of these herbs and foods should discuss possible interactions with their pharmacist, physician, or other qualified health care professional.

—Stefan Gafner, PhD


  1. Why herbal supplements taken with prescription drugs may be risky. CBS News. January 24, 2018. Available at: Accessed February 14, 2018.
  2. Herbal products may compromise prescription drugs and cause serious side effects. ScienceDaily. January 24, 2018. Available at: Accessed February 14, 2018.
  3. Macmillan A. Herbal supplements may be dangerous when you take certain prescription drugs. TIME. January 24, 2018. Available at: Accessed February 14, 2018.
  4. Awortwe C, Makiwane M, Reuter H, Muller C, Louw J, Rosenkranz B. Critical evaluation of causality assessment of herb-drug interactions in patients. British Journal of Clinical Pharmacology. Published online January 24, 2018. Available at: Accessed February 14, 2018.
  5. Gurley BJ, Fifer EK, Gardner Z. Pharmacokinetic herb-drug interactions (part 2): Drug interactions involving popular botanical dietary supplements and their clinical relevance. Planta Med. 2012;78(13):1490-1514.
  6. Sprouse AA, van Breemen RB. Pharmacokinetic interactions between drugs and botanical dietary supplements. Drug Metab Dispos. 2016;44(2):162-171.
  7. Gurley BJ. Pharmacokinetic herb-drug interactions (part 1): Origins, mechanisms, and the impact of botanical dietary supplements. Planta Med. 2012;78(13):1478-1489.