Dietary supplements can have serious side effects when mixed with prescription drugs, but not all herb-drug interactions are bad.

Jul 1, 2012
Catherine Ulbricht


For more than 5,000 years, herbs and other natural ingredients have been used for medicinal purposes. Today, people use such concentrated natural products as supplements to help combat various diseases, from depression to cancer, as well as to boost health, including immunity and memory. Based on Natural Standard research, in the United States alone more than $40 billion is spent each year on these products. An estimated 60 percent of cancer patients try natural products, and 40 percent take vitamins or other dietary supplements.

Just because herbal products are developed from plants, they cannot necessarily be deemed harmless. Like prescription drugs, herbs and supplements may cause unwanted side effects and can interact with prescription drugs, other natural products, or foods, and may even alter diagnostic and laboratory test results. Unlike regulated drugs, however, dietary supplements can be marketed without approval from the US Food and Drug Administration. As a result, herbal products are often not thoroughly evaluated by the FDA unless there is sufficient evidence to prove that they are unsafe. Partly due to this regulatory freedom, as well as to a lack of available clinical research, interactions between herbs and conventional drugs are often overlooked.

In the United States alone more than $40 billion is spent each year on natural products.

Herbs and drugs can interact pharmacodynamically by mechanisms that may be additive, synergistic, or antagonistic. For example, concurrent use of an anticoagulant/antiplatelet drug and natural ingredients that possess antiplatelet activity, such as garlic, may increase the risk of bleeding. Similarly, herbs that lower blood sugar may have additive effects with antidiabetic drugs, thereby increasing the risk of potentially dangerous hypoglycemia (low blood sugar). Some herbs, such as ephedra (ma huang in Chinese), on the other hand, are known to increase blood pressure and may counteract the beneficial effects of antihypertensive medications.

Herbs and medications can also have pharmacokinetic interactions, meaning that the herbs may change the absorption, distribution, metabolism, or excretion of a drug, resulting in altered effects.  Herbs that alter gastrointestinal function, for example, can affect drug absorption, as can those that induce or inhibit metabolic enzymes and transport proteins. Many other herbs have been identified as substrates, inducers, and/or inhibitors of the liver’s cytochrome P450 enzyme system, which is extensively involved in drug metabolism, and can thus affect the clearance of drugs. St. John’s wort, popularly used for the treatment of depression, may reduce levels of antiretroviral agents and immunosuppressants, and can increase the neurotransmitter serotonin, thus making it dangerous to combine with drugs that affect serotonin levels, such as selective serotonin reuptake inhibitor (SSRI) antidepressants.

Many of these interactions could be prevented simply by taking the drug so many hours before or after the supplement, for instance. Furthermore, not all interactions are bad. Thus, combining dietary supplements and prescription drugs should not necessarily be discouraged, but clinicians need to be aware of the potential risks. Indeed, more than 75 percent of doctors and nurses now seek information on complementary and alternative medicine each year, and these numbers are growing. Given this burgeoning interest, it is important to develop and maintain credible resources on herbs and supplements, such as the Natural Standard Research Collaboration, of which I am cofounder. On our website (www.naturalstandard.com), clinicians can access databases and charts guiding appropriate dosage, as well as details about potential concerns. The collaboration also offers continuing medical-education courses dedicated to integrative medicine.

Until health-care providers begin to be more aware of potential drug-herb interactions and begin to utilize the resources available to them to avoid damaging side effects, dietary supplement-drug mixology will remain problematic. 

Catherine Ulbricht is a senior attending pharmacist at Massachusetts General Hospital, and editor-in-chief of both the Journal of Dietary Supplements and Natural Standard.