Older adults at high risk for drug interactions

At least one in 25 older adults, about 2.2 million people in the United States, take multiple drugs in combinations that can produce a harmful drug-drug interaction, and half of these interactions involve a non-prescription medication, researchers from the University of Chicago Medical Center report in the Dec. 24/31, 2008, issue of JAMA.

Although the number of people taking medications has remained stable for the last decade, the number of drugs taken by older people has significantly increased. This may be because of more intense therapy for chronic illness, improved access to medications due to Medicare Part D, and the growth of the generic drug market. More than half of older adults now take five or more medications or supplements.

"Older adults are the largest consumer of prescription drugs," said study author Stacy Tessler Lindau, MD, assistant professor of obstetrics and gynecology and of medicine at the University of Chicago Medical Center. "We find that they commonly combine these prescription medications with over-the-counter medications and dietary supplements, which can increase their vulnerability to medication side-effects and drug-drug interactions."

"We were reassured that we did not find combinations of the most commonly used drugs that were absolutely forbidden," she added, "one indication that drug safety systems used by physicians, nurses and pharmacists are working."

"However," she added, "our results probably underestimate total risk." Patients using less common drugs and non-prescription medications could be more at risk for harmful interactions because health care providers may be less familiar with their safety profile.

The study also found ethnic and gender differences. Older Hispanics were more likely than other ethnic groups to be taking no medications. Older women were less likely than older men to take medicines to reduce cholesterol.

"In our study, men and women were equally likely to report a history of cardiovascular disease," said co-author Dima M. Qato, PharmD, MPH, research associate in obstetrics and gynecology at the University. Despite efforts to increase awareness in the medical community that older men and women are equally at risk for a cardiovascular event, "disparities persist in the use of statin medications," she said. "Far fewer women than men were taking these effective cholesterol-lowering drugs."

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 administered between July 2005 and March 2006. The survey team interviewed 3005 participants in their homes about the medications they used "on a regular schedule, like every day or every week." Ninety-nine percent, 2,976 respondents, completed the interview and medication log.

Ninety-one percent of all respondents regularly used at least one medication, a percentage that increased with age. Twenty-nine percent of older adults took more than five prescription medications.

Sixty-eight percent of the adults who took prescription drugs also used over-the-counter medications or dietary supplements. Men were more likely to take over-the-counter medicines. Women were more likely to use supplements, such as vitamins or herbal remedies.

Nearly half of the drug-drug interactions identified could cause bleeding problems. One of the most common was taking 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

"Physicians and pharmacists need to ask their patients about the use of nonprescription medications," said Lindau. "Patients need to inform their providers about all medications they use--prescription and nonprescription--and should ask their physician or pharmacist about interactions any time they start a new drug, on their own or following the doctor's recommendation."

Carrying a list of all medications in a wallet might be a good idea, the authors suggest. So is using the same pharmacy or chain for all medications, said Qato. The researchers also stressed that health professionals need to stay on top of drug-safety information and consider the evidence, where available, for safety in older adults.

The National Institutes of Health and the University of Chicago Program on Pharmaceutical Policy funded the study. Additional authors include Caleb Alexander, Rena Conti, Michael Johnson, and Phil Schumm of the University of Chicago.