U. S. researchers said that as many as 4 % of the older adults in the United States are combining over the counter medications with prescription drugs in combinations that could prove to be deadly.

The study was undertaken by Dr. Stacy Tessler Lindau of the University of Chicago Medical Center in Illinois with Dima Qato, a pharmacist and researcher at the University of Chicago. Lindau said, "Older adults in the United States use medicine and they use a lot of it. While medications are often beneficial, they are not always safe."

"The vast majority of older adults are using at least one medication and more adults are using more medications, particularly prescription medications, compared to a decade ago," said Qato.

The researchers said at least 30 % of the older adults are taking at least five prescription medications and many combine the prescription drugs with nonprescription drugs. "And despite limited availability of drug safety information for nonprescription medications, particularly dietary supplements, they are frequently used in older adults," Qato added.

The researchers used data from a national survey of adults aged 57 to 85 and interviews with nearly 3,000 people in their homes to find out about the medications they used on a regular basis which included both over-the-counter and prescription drugs, used between June 2005 and March 2006.

They found that 91 % of U. S. adults used at least one medication regularly while prescription medicines were used by 81 % of adults. 68 % of adults surveyed who took prescription drugs also used over-the-counter drugs or dietary supplements.

In the highest risk were men in the 75 to 85 year bracket with "One in 10 men between the ages of 75 to 85 were at risk for a drug-to-drug interaction," said Qato.

In addition, almost 50 % of older adults used at least one over-the-counter medication or dietary supplement while more women than men used prescription medications and dietary supplements.

The survey also showed that more than 50 % of those surveyed used five or more prescription drugs, over-the-counter medications or dietary supplements.

"If they need to self-medicate with over-the-counter drugs or dietary supplements, they should consult with their physician or pharmacist. This is particularly important in older people because as people get older, they are more vulnerable to the negative effects of medications, including drug-to-drug interactions," Qato said.

The most common drug to drug reaction is bleeding problems and the blood thinner warfarin, often sold by Bristol-Myers Squibb Co. under the brand name Coumadin, was most commonly cited in potentially dangerous combinations. Ginkgo biloba, a popular nutritional supplement taken in combination with aspirin was also a potential cause of bleeding.

"Physicians and pharmacists need to ask older patients about all the medications they use -- prescription and nonprescription -- and patients need to be prepared to share this information," Qato said. "This is especially important in patients who see multiple providers and patients that fill at multiple pharmacies."

Dr. Laurie Jacobs, chief of geriatric medicine at Montefiore Medical Center in New York City said, "No one is looking over the number of medicines they are on to look for potential interactions. Someone other than the patient should go over the whole list. Often patients have difficulty evaluating the appropriateness of supplements with their own medical problems," Jacobs said. "They are not seeking the advice of their physician on supplement use," she said.

The report was published in the Journal of the American Medical Association.