More than half of older Americans taking an antidepressant for the first time were already taking another drug that could interact with it and cause side-effects, researchers reported on Friday.
And a quarter of patients who suffered side-effects stopped taking antidepressants altogether, the study by a team at Thomson Reuters, the University of Southern California, Sanofi Aventis and elsewhere found.
"We found a concerning degree of potentially harmful drug combinations being prescribed to seniors," Tami Mark of Thomson Reuters, parent company of Reuters, said in a statement.
Other studies have found that older adults are often taking dangerous combinations of prescription drugs, but doctors are not getting the message, the researchers reported in the American Journal for Geriatric Psychiatry.
The research team used a Thomson Reuters database of claims for Medicare, the federal health insurance plan for people over 65.
They found more than 39,000 patients who started antidepressants between 2001 and 2006. "Twelve commonly reported antidepressant side effects were identified in the month after drug initiation," Mark's team wrote.
More than 25 percent of the patients were prescribed antidepressants and another drug that could cause a major interaction. Another 36 percent had potential moderate interactions.
"The most common side effect was insomnia, somnolence, and drowsiness, which occurred in 1,028 (2.6 percent) patients. The next most common side effect was dizziness, which was documented in 416 (1.1 percent) patients," the researchers wrote.
The side-effects meant patients often dropped the drug they were taking. Only 45 percent of those with documented side-effects refilled the prescription for the same antidepressant and a quarter quit taking antidepressants altogether.
Many adults are at risk of this problem, the researchers noted — other studies show that 25 percent of older adults with chronic illnesses such as arthritis or heart disease also have depression, and they have also been shown to be helped by antidepressants.
"Older adults often need to be on many medications, some of which may contribute to depression and/or interact with antidepressants. Finally, older adults metabolize medications slowly and are more sensitive to side effects than younger patients," the researchers concluded.