Herbs: Toxicities and Drug Interactions

Are herbal remedies and supplements safe to take?

Any conventional medication can have side effects. These side effects are described and reported after drug trials and research studies have been conducted. Side effects are further reported and evaluated after the marketing of the medication. Information about drug components, interactions, usage in pregnancy, while breastfeeding, for pediatric patients, and dosing limits are outlined and made available in standard references for doctors treating patients. Furthermore, the formulations of the drugs must satisfy strict quality control standards to ensure conformity. These medications regularly contain virtually uniform quantities and ratios of substances.

In contrast to conventional medications, unconventional treatments (such as herbs) have little or no actual scientific basis so doctors can guide their patients regarding proper usage or potential toxicity. There are no standardized references and most of the herbal formulations have not been analyzed, are not uniform, and have not been quality controlled. One batch can be very different from the next.

Moreover, even if a given herb has a known toxicity, the manufacturer may or may not warn consumers. Manufacturers are not required to alert consumers to known dangers.

The point is that these "supplements" are not sanctioned, regulated, or supervised by any agency.

Data is coming, although slowly. Dr. Lucinda Miller of Texas Tech University Health Sciences reviewed known herb-drug interactions. Her review was published for doctors in the medical journal Archives of Internal Medicine. The list that follows is derived from this article and includes summaries of various herbs with particular focus on potential herb-drug interactions.

Keep in mind that the information in the "Uses" section is for the most part unsupported by verification of scientific studies. It should be noted that simply because herbs are "natural" treatments, they are not necessarily free from side effects.

Chamomile drug interactions

  • Uses: Chamomile is often used in the form of a tea as a sedative.
  • Drug interactions: Allergic reactions can occur, particularly in persons allergic to ragweed. Reported reactions include abdominal cramps, tongue thickness, tightness in the throat, swelling of the lips, throat and eyes, itching all over the body, hives, and blockage of the breathing passages. Close monitoring is recommended for patients who are taking medications to prevent blood clotting (anticoagulants) such as warfarin (Coumadin, Jantoven).

Echinacea drug interactions

  • Uses: Largely because white blood cells in the laboratory can be stimulated to eat particles, Echinacea has been touted to be able to boost the body's ability to fight off infection.
  • Drug interactions: The most common side effect is an unpleasant taste. Echinacea can cause liver toxicity. It should be avoided in combination with other medications that can affect the liver such as ketoconazole (Nizoral, Extina, Xolegel, Kuric), leflunomide (Arava), methotrexate (Rheumatrex, Trexall), isoniazid (INH, Nydrazid, Laniazid).

St. John's Wort drug interactions

  • Uses: St. John's Wort is popularly used as an herbal treatment for depression, anxiety, and sleep disorders. It is technically known as Hypericum perforatum. Chemically, it is composed of at least 10 different substances that may produce its effects. The ratios of these different substances varies from plant to plant (and manufacturer).
  • Drug interactions: The most common side effect has been sun sensitivity, which causes burning of the skin. It is recommended that fair-skinned persons be particularly careful while in the sun. St. John's wort may also leave nerve changes in sunburned areas. This herb should be avoided in combination with other medications that can affect sun sensitivity such as tetracycline/Achromycin, sulfa- containing medications, piroxicam (Feldene). St. John's wort can also cause headaches, dizziness, sweating, and agitation when used in combination with serotonin reuptake inhibitor medications (SSRIs) such as fluoxetine (Prozac) and paroxetine (Paxil).

Garlic Drug interactions

  • Uses: Garlic has been used to lower blood pressure and cholesterol (Dr. Lucinda Miller notes that there is "...still insufficient evidence to recommend its routine use in clinical practice.")
  • Drug interactions: Allergic reactions, skin inflammation, and stomach upset have been reported. Bad breath is a notorious accompaniment. Studies in rats have shown decreases in male rats' ability to make sperm cells. Garlic may decrease normal blood clotting and should be used with caution in patients taking medications to prevent blood clotting (anticoagulants) such as warfarin (Coumadin).

Feverfew drug interactions

  • Uses: Most commonly used for migraine headaches.
  • Drug interactions: Feverfew can cause allergic reactions, especially in persons who are allergic to chamomile, ragweed, or yarrow. Nonsteroidal anti-inflammatory drugs NSAIDs such as ibuprofen (Advil), naproxen (Aleve) or Motrin can reduce the effect of feverfew. A condition called "post-feverfew syndrome" features symptoms including headaches, nervousness, insomnia, stiffness, joint pain, tiredness, and nervousness. Feverfew can impair the action of the normal blood clotting element (platelets). It should be avoided in patients taking medications to prevent blood clotting (anticoagulants) such as warfarin (Coumadin).

Ginkgo Biloba drug interactions

  • Uses: This herb is very popular as a treatment for dementia (a progressive brain dysfunction) and to improve thinking.
  • Drug interactions: Mild stomach upset and headache have been reported. Ginkgo seems to have blood thinning properties. Therefore, it is not recommended to be taken with aspirin, nonsteroidal anti-inflammatory drugs such as ibuprofen (Advil),  naproxen (Aleve) or Motrin, or medications to prevent blood clotting (anticoagulants) such as warfarin (Coumadin). Ginkgo should be avoided in patients with epilepsy taking seizure medicines, such as phenytoin  (Dilantin), carbamazepine (Tegretol), and phenobarbital.

Ginseng drug interactions

  • Uses: Ginseng has been used to stimulate the adrenal gland, and thereby increase energy. It also may have some beneficial effect on reducing blood sugar in patients with diabetes mellitus. (Dr. Miller emphasized that there is substantial variation in the chemical components of substances branded as "Ginseng.")
  • Drug interactions: Ginseng can cause elevation in blood pressure, headache, vomiting, insomnia, and nose bleeding. Ginseng can also cause falsely abnormal blood tests for digoxin (Lanoxin) levels in persons taking the drug for heart disease. It is unclear whether ginseng may affect female hormones. Its use in pregnancy is not recommended. Ginseng may affect the action of the normal blood clotting element (platelets). It should be avoided in patients taking aspirin, nonsteroidal anti-inflammatory drugs such as ibuprofen (Advil), naproxen (Aleve) or Motrin, or medications to prevent blood clotting (anticoagulants) such as warfarin (Coumadin). Ginseng may also cause headaches, tremors, nervousness, and sleeplessness. It should be avoided in persons with manic disorder and psychosis.

Ginger drug interactions

  • Uses: Ginger has been used as a treatment for nausea and bowel spasms.
  • Drug interactions: Ginger may lead to blood thinning. It is not recommended to be taken with medications that prevent blood clotting (anticoagulants) such as warfarin (Coumadin).

Saw Palmetto drug interactions

  • Uses: Saw palmetto has been most commonly used for enlargement of the prostate gland. Saw palmetto has also been touted as a diuretic and urinary antiseptic to prevent bladder infections. However, according to the National Institutes of Health (NIH), there is insufficient scientific evidence to support the use of saw palmetto for treatment of enlarged prostate or any other conditions.
  • Drug interactions: Saw palmetto can cause upset stomach. This herb may affect the action of the sex hormone testosterone, thereby reducing sexual drive or performance. Dr. Miller states that "While no drug-herb interactions have been documented to date, it would be prudent to avoid concomitant use with other hormonal therapies (for example, estrogen therapy and oral contraceptives...")

This listing represents only a small portion of herbal treatments. Nevertheless, the popularity of herbal therapies is unquestionable. Doctors routinely confront the unknown with their patients who are using herbs. Doctors simply do not have any way of helping you to decide whether these herbs are helpful or harmful for you, or whether they are interacting with your current medications. There are no data.

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Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care


L. Miller. "Herbal Medicinals." Archives of Internal Medicine 1998;158:2200-2209.

UpToDate. Clinical use of saw palmetto.