About 20 percent of older Americans take multiple medications that may interfere with each other, a study published Feb. 25 in PLOS ONE found. Despite the frequency of this “therapeutic competition,” providers change treatments only about 16 percent of the time.

Songprod Jonathan Lorgunpai, a medical student at Yale University School of Medicine in New Haven, Conn., and co-investigators evaluated data from a cohort of 5,815 adults 65 and older living in the community who participated in the Medicare Current Beneficiary Survey. Using claims data, they determined the 14 most common chronic conditions treated with at least one medication. They identified the medications recommended by national guidelines and used by at least 2 percent of the participants through interviews.

Potential therapeutic competition was defined as either a well-known adverse effect of a medication, a mention in the guidelines or a report in a systematic review or two studies published since 2000. As outcomes, they determined how frequently therapeutic competition may have occurred and how often the participants with and without the condition used the medication.

Fifteen of 27 medication classes (55.5 percent) recommended for a medication investigated in the study could have adverse effects on other conditions. Of 91 pairs of study conditions, 25 (27.5 percent) had at least one potential therapeutic competition. Among the pairs they identified were hypertension and osteoarthritis; hypertension and diabetes; and diabetes and coronary artery disease.

There were 1,313 participants (22.6 percent) who took at least one medication that could worsen another condition and 753 (13 percent) had more than one pair of these conditions.Despite the prevalence of potential therapeutic competition, providers changed drugs in only 16.2 percent of the cases.

The authors also noted that studies typically do not evaluate the harms and benefits of drugs in relation to coexisting conditions and there are few guidelines addressing the potential competitive adverse effects of medications.

“In addition to considering the effect of medications on coexisting conditions, heightened awareness of therapeutic competition should trigger systematic attention to identifying strategies for avoiding poor clinical outcomes in individuals with competing chronic conditions,” they wrote.