Nearly half of all older adults use herbal and dietary supplements regularly, yet most fail to share that information with their doctors, according to researchers at the Intermountain Medical Center in Salt Lake City.

Unfortunately, this oversight can lead to problems with prescription drugs, especially drugs that are highly sensitive and easily thrown off balance. These drugs have what's called a narrow safety margin and include medications such as digoxin, lithium, phenobarbital and warfarin, among others.

Popular herbal and dietary supplements can interact with these medications and alter the way they work in the body, making the drugs either more or less effective, or increasing certain side effects.

The Utah researchers were particularly interested in the commonly used blood thinner warfarin (Coumadin). Certain supplements either intensify its effect and increase the risk of bleeding because blood does not clot well — or decrease its effectiveness enough so that a clot develops, which may cut off part of the blood supply to the brain and cause a stroke.

To find out how many people taking warfarin also used supplements, the researchers surveyed 100 men and women on their first visit to a service that monitors warfarin therapy. They asked about supplement use, whether their doctors brought up the subject and whether the patients told their doctors about supplement use without being asked.

They found that 69 of the 100 men and women surveyed used supplements. The five most popular included multivitamins; individual vitamins; glucosamine, condroitin or a combination of the two; fish oil; and coenzyme Q10. Only one-third of the group said their doctors questioned them about supplement use, yet almost all reported that they would certainly own up if asked. About half didn't feel that supplements were drugs and a majority did not consult their doctor or pharmacist before starting on one.

"Many people think that supplements aren't really medicine since they don't require a prescription and they aren't listed in the drugstore's over-the-counter medication area," says Harvard Medical School cardiologist Elliott Antman, M.D. "They don't even have them on their medicine radar screen, so they don't mention it to their doctors because they think it's irrelevant. This is a real problem."

Take the hypothetical example of Mrs. M, a 70-year-old woman with chronic atrial fibrillation (fast, erratic heartbeat). She's taking warfarin to reduce the risk of a stroke and also taking several medications to control high blood pressure.