BAKERSFIELD, Calif. -- More than 15 million Americans use herbal remedies or megavitamins in the hopes of improving their health.

Doctors from the Mayo Clinic have warned that these popular supplements may interact with prescription medications in ways that increase heart risk for patients.

For example, St. Johns Wort can make blood pressure and cholesterol pills less effective. Gingko biloba -- which many people take to keep their minds sharp -- can interact with Coumadin or aspirin to increase the risk for internal bleeding.

Even high doses of garlic have the potential to raise bleeding risk when taken with anti-clotting drugs.

The researchers said part of the problem is that patients do not tell their doctors when they are taking these over-the-counter supplements, so physicians don't realize they might be placing their patients at risk.

To avoid drug interactions of any kind, it's a good idea to bring a list of all your medications to any doctor's appointments, including herbals, vitamins, and over-the-counter drugs such as Tylenol or Benadryl.