According to researchers in the United States about 2.2 million seniors put themselves at high risk for drug interactions - they say many non-prescription medication, which are bought over-the-counter (OTC) can produce a harmful drug interactions.
The researchers from the University of Chicago Medical Center say more attention is needed for non-prescription medications and dietary supplements as at least one in 25 older adults take multiple drugs in combinations that can be dangerous.
According to the researchers while the number of people taking medications has remained stable for the last decade, the number of drugs taken by older people has significantly increased - they say this may be because of more intense therapy for chronic illness, improved access to medications and the growth of the generic drug market.
Study author Dr. Stacy Tessler Lindau, an assistant professor of obstetrics, gynaecology and medicine, says more than half of older adults now take five or more medications or supplements and older adults are the largest consumer of prescription drugs.
Dr. Lindau says these prescription medications are often combined with over-the-counter medications and dietary supplements, which can increase their vulnerability to medication side-effects and drug-drug interactions.
Dr. Lindau says though drug safety systems used by physicians, nurses and pharmacists are working and combinations of the most commonly used drugs that are absolutely forbidden, were not found, the risk has probably been underestimated.
The study also found ethnic and gender differences with older Hispanics more likely than other ethnic groups to be taking no medications and older women less likely than older men to take medicines to reduce cholesterol, even though men and women were equally likely to report a history of cardiovascular disease.
The study used data collected for the National Social Life, Health and Aging Project, a nationally representative multi-purpose survey of adults aged 57 to 85 carried out between July 2005 and March 2006.
The survey team interviewed 3005 participants in their homes about the medications they used on a regular basis and it was found that 91% of all respondents regularly used at least one medication, and that percentage increased with age, and 29% of older adults regularly took more than five prescription medications.
Of the adults who took prescription drugs 68% also used over-the-counter medications or dietary supplements.
Almost 50% of the drug-drug interactions identified could cause bleeding problems, the most common was Warfarin, a prescription drug designed to prevent blood clots, along with an over-the-counter drug such as aspirin, which also interferes with clotting.
The most common potentially severe medication interactions with prescription drugs included Lisinopril, Warfarin, Atorvastatin, Simvastatin - non-prescription preparations included Ginkgo and aspirin.
Around 2 million Americans are prescribed Warfarin following a heart attack, stroke or major surgery and the team say patients need to know about these risks and they recommend patients carry a list of all of the drugs and supplements they take.
They also say doctors, pharmacists and other health professionals should remember to ask patients about all of the medications they are taking.
The study appears in the Journal of the American Medical Association.