In fact, the agency advised in 2016 that fluoroquinolones’ potential side effects outweighed their benefits for several common infections. Last year, it added still another warning about ruptures or tears in the aorta, a rare but serious condition for which older people are at greater risk.

Fluoroquinolones are also most implicated in the rampant, difficult-to-cure infection called C. difficile, along with an earlier antibiotic, clindamycin. C. difficile, too, occurs more frequently in older people.

Yet what class of antibiotics did the C.D.C. team determine was most commonly prescribed for older adults? Fluoroquinolones. (The most used single drug was azithromycin, marketed as Zithromax, which isn’t a quinolone.)

More troublingly, doctors often prescribe these medications unnecessarily, studies repeatedly show. Upper respiratory infections — colds, sinus infections, bronchitis — trigger most prescriptions, but those infections are typically viral, not bacterial, and thus impervious to antibiotics.

Nonetheless, a large 2017 study of older adults in Ontario found that almost half were prescribed antibiotics for nonbacterial upper respiratory infections that likely would have cleared up in a few days without them. “Patients usually get better in spite of the drugs, not because of them,” Dr. Alexander said.

As Dr. Kabbani pointed out, “when antibiotics are needed, they are lifesaving drugs.” But because they are so widely misused, with resistance such a menace, the C.D.C. has pushed for more prudent practices for consumers and for hospitals. The Centers for Medicare and Medicaid Services, similarly, is phasing in policies for wiser antibiotic use in nursing homes.

“We want patients to get antibiotics when they need them — the right drug at the right time and the right dose — and not when they don’t,” said Dr. Fleming-Dutra. Even when antibiotics prove necessary, she noted, patients may get the wrong ones or take them for too long.