Many seniors take at least one prescription drug on a regular basis, and from time to time they might combine their routine pill with another over-the-counter or prescription med. While no one knows what combining two "safe" drugs may do, in at least one case the practice could be fatal. A new study finds the interaction of an antibiotic with a diuretic more than doubles the risk of death for older patients.

The antibiotic trimethoprim-sulfamethoxazole is prescribed by many doctors for patients suffering from urinary tract and other infections. In fact, every year doctors write more than 20 million scripts of this drug in the United States alone. Another drug, spironolactone frequently is prescribed to treat fluid retention in the elderly who have been diagnosed with congestive heart failure (and other conditions, such as cirrhosis of the liver). This common diuretic also is used to treat or prevent low potassium levels in the blood.

For the current study, researchers tracked 206,319 elderly patients taking spironolactone between April 1994 and December 2011 — a 17-year period. All of the patients were over the age of 66, living in Ontario, and so had universal access to physician services and prescription drug coverage. Of these patients on spironolactone, 11,968 died suddenly and 328 died within 14 days after taking either trimethoprim-sulfamethoxazole, amoxicillin, ciprofloxacin, norfloxacin, or nitrofurantoin.

"Sudden death during spironolactone treatment was more than twice as likely following a prescription for trimethoprim-sulfamethoxazole than for amoxicillin," Dr. Tony Antoniou of the Li Ka Shing Knowledge Institute, St. Michael's Hospital, and his co-authors conclude in their published study.

Drugs & the Elderly

More than three-quarters of all seniors between 65 and 79 are suffering from one or more chronic disease, while the same is true for 85 percent of seniors over the age of 80. Naturally, then, seniors use more prescription and over-the-counter drugs than younger people. A 2013 report from the Henry J. Kaiser Family Foundation estimates the average senior over than 65 fills 27 prescriptions each year. Unsurprisingly, a much older report (1996) from the Food and Drug Administration found up to a quarter of drug use in seniors is considered unnecessary or inappropriate, while 28 percent of elderly hospitalizations are caused by adverse drug reactions and noncompliance.

The Internet is rich in advice on how the elderly might use prescription drugs safely. However, first and foremost you must understand this one significant point, raised by many, though most convincingly by Dr. Mercola: While the FDA may test the safety of individual drugs, they rarely, if ever, evaluate the safety of multiple drugs taken together. It's anyone's guess what will happen if you mix drugs, whether they be over-the-counter medications or prescribed pills.

Source: Antoniou T, Hollands S, Macdonald EM, et al.Trimethoprim–sulfamethoxazole and risk of sudden death among patients taking spironolactone. CMAJ. 2015.