Among older men, the numbers are particularly alarming - one in 10 are taking potentially harmful combinations, according to the study.
The results aren't always disastrous, but older people are more vulnerable to side effects and drug-to-drug interactions. And patients need to know that just because lots of medicines and supplements don't require prescriptions doesn't mean they're harmless. Nor are some of these safe to take when you're prescribed other medications.
Experts say the take-home advice is to ask about any side effects of prescription drugs, and tell your doctor before taking other medicines.
The report showing just how many older people are using risky combinations comes from a study of nearly 3,000 interviews with people aged 57 to 85. The research, funded by the National Institutes of Health and University of Chicago, appears in Wednesday's Journal of the American Medical Association.
It's well-known that as people age, their medication use goes up. But the study highlights the potential problems with some sobering statistics.
Ninety-one percent in this age group use at least one medication, often for heart disease and related problems. That translates to more than 50 million people. More than half use at least five remedies, including prescriptions, over-the-counter medicines or supplements.
Virtually every medicine can have side effects, and with so many being used by so many older adults, the potential for harm is high.
For example, warfarin, a potent prescription clot-fighting drug, was often taken with aspirin. Both increase the risk of bleeding, so the odds are even higher when both drugs are taken. The researchers said these risks also occur when warfarin is taken with garlic pills, which some studies have suggested can benefit the heart and help prevent blood clots.
Signs of bleeding problems include bruising easily, hard-to-stop bleeding from the gums or from cuts and blood in the urine.
Other commonly used and risky combinations included:
Aspirin taken with over-the-counter ginkgo supplements, increasing chances for excess bleeding.
Lisinopril, a blood pressure drug, taken with potassium, which combined can cause abnormal heart rhythms. Potassium is often prescribed to restore low levels of this important mineral caused by certain blood pressure drugs.
Prescription cholesterol drugs called statins taken with over-the-counter niacin, a type of vitamin B that also lowers cholesterol. This combination increases risks for muscle damage.
"Patients need to know that while medications are often beneficial, they're not always safe," said lead author Dima Qato, a University of Chicago pharmacist. "If they need to self-medicate with over-the-counter or dietary supplements, they should definitely consult with their physicians or pharmacists."
The study relied on data from in-person interviews with 2,976 adults questioned about which medications they routinely used. The nationally representative survey was done between June 2005 and March 2006, and results were extrapolated to the general population.
The researchers assessed how many people routinely used at least two medications of any type known to have dangerous or even fatal interactions. The number totaled at least one in 25, corresponding to 2.2 million nationwide.
Those interviewed weren't asked if they'd ever had a bad reaction from taking those combinations. And the study didn't assess whether patients were taking medications inappropriately.
Dr. Jerry Gurwitz, chief of the geriatric medicine division at University of Massachusetts Medical School, said taking multiple medications, despite possible bad interactions, isn't necessarily a bad idea as long as patients are in close contact with their doctor.
"There are definitely many instances where if they're monitored carefully and there's good reason for using them, that they could be used safely," said Gurwitz, who wasn't involved in the study.
Prescription drugs were the most commonly used and nearly one-third used at least five prescription drugs.
Michael Cohen, a pharmacist and president of the Institute for Safe Medication Practices, called the study an important snapshot of medication use in older Americans. But as someone who takes at least five medications himself, Cohen said the widespread prevalence isn't surprising.
Cohen said his group recently launched a new Web site that will allow consumers to enter names of their medications to check for any potentially dangerous interactions.