Dietary supplements can alter the absorption and metabolism of prescription and over-the-counter medications, the FDA said.
"Some dietary supplements may increase the effect of your medication, and other dietary supplements may decrease it," Robert Mozersky, a medical officer at the FDA, explained in an agency news release.
For example, the supplement St. John's Wort can make birth control pills less effective, the FDA reported. Both the herbal supplement ginkgo biloba and vitamin E can thin blood. Mixing either supplement with the prescription blood thinner warfarin or aspirin could increase the risk of internal bleeding or stroke, the report said.
Dietary supplements are widely used in the United States. The U.S. Centers for Disease Control and Prevention's 2005-2008 National Health and Nutrition Examination Survey found that roughly 72 million people in the United States who are on a prescription medication also took some type of dietary supplement.
Although many people take supplements to make sure they get proper nutrition, the FDA said there is no substitute for eating a healthy diet, and products labeled as "natural" or "herbal" are not necessarily harmless.
"Natural does not always mean safe," Mozersky said. This is particularly true for children, he added.
"Parents should know that children's metabolisms are so unique, that at different ages they metabolize substances at different rates. For kids, ingesting dietary supplements together with other medications make adverse events a real possibility," Mozersky explained.
People planning to have surgery should inform their doctor of every medication and supplement they use. It may be necessary to stop taking supplements a few weeks before an operation to avoid potentially serious changes in heart rate, blood pressure or bleeding risk, the FDA said.
Women who are pregnant or breast-feeding should also talk to their doctor about any supplements they take.
"The bottom line is, before you take any dietary supplement or medication -- over-the-counter or prescription -- discuss it with your health care professional," Mozersky said. The FDA added the following tips for consumers:
- Every time you visit the doctor, bring a list of all the dietary supplements and medications you take. This list should include dosages and frequency.
- Tell your doctor if your health has changed, including any recent illnesses, surgeries or other procedures. You should also tell your doctor if you are pregnant or breast-feeding.
The U.S. National Center for Complementary and Alternative Medicine has more about the safe use of herbal supplements.