CreditJun Cen

Last month, Caryn Isaacs went to see her primary care doctor for her annual Medicare wellness visit. A patient advocate who lives in Manhattan, Ms. Isaacs, 68, felt perfectly fine and expected a clean bill of health.

But her doctor, who’d ordered a variety of blood and urine tests, said she had a urinary tract infection and prescribed an antibiotic.

“The nurse said, ‘Can you take Cipro?’” Ms. Isaacs recalled. “I didn’t have any reason not to, so I said yes.”

There are actually plenty of reasons for older people to avoid Cipro and other antibiotics known as fluoroquinolones, which have prompted warnings from the Food and Drug Administration about their risks of serious side effects.

And there are good reasons to avoid any antibiotic when bacteria is detected in a urine culture in a patient who has no other signs of infection. So-called asymptomatic bacteriuria increases with age, but these women are not sick and don’t need drugs, so medical guidelines recommend against routine screening or treatment.

Yet Ms. Isaac’s prescription was hardly unusual. Despite ongoing campaigns by the Centers for Disease Control and Prevention and other public health groups, older Americans still take too many antibiotics.

Patients over age 65 have the highest rate of outpatient prescribing of any age group. A new C.D.C. study, published in the Journal of the American Geriatrics Society, points out that doctors write enough antibiotic prescriptions annually — nearly 52 million in 2014 — for every older person to get at least one.

Because the researchers used a national pharmacy database that tracked only outpatients, the study likely underestimates the problem. “The volume would be higher if you included hospitals and nursing homes and other long-term care settings,” said Katherine Fleming-Dutra, deputy director of the C.D.C.’s Office of Antibiotic Stewardship.

Glass-half-full types might be pleased to see that after climbing 30 percent from 2000 to 2010, antibiotic prescriptions for older adults leveled off between 2011 and 2014. “That’s potentially good news,” said Dr. Sarah Kabbani, an infectious disease specialist at the C.D.C. and lead author of the study.

But what public health advocates want to see is a decline, as has happened with young children, once the group most likely to use antibiotics.

“It’s hard to feel heartened about a plateau when overuse remains so prevalent,” said Dr. Caleb Alexander, co-director of the Johns Hopkins Center for Drug Safety and Effectiveness. “It’s as perennial as the grass.”

Antibiotic overuse contributes to a serious public health threat by creating drug resistance, as infectious bacteria adapt to the medications. Drugs then lose their effectiveness, forcing doctors to resort to more toxic, less potent, often costlier options. Two million Americans get antibiotic-resistant infections annually, the C.D.C. has reported, and 23,000 die from them.

Moreover, antibiotics interact badly with many of the other drugs older adults take, including such widely used medications as statins, blood thinners, kidney and heart medications. “The number of potential drug-drug interactions with antibiotics are vast,” Dr. Alexander cautioned.

Some antibiotics also have dismaying, even alarming, side effects in themselves. In 2013, the F.D.A. issued a warning about azithromycin, which in rare cases leads to dangerous heart arrhythmias.

But for more than a decade, the agency’s most frequent target has been fluoroquinolones.

It has warned that this class of antibiotics (including Cipro and Levaquin) increases the risk of tendinitis and tendon rupture, particularly in older adults; that it can cause the nerve damage called peripheral neuropathy; and that it can lead to hypoglycemia (low blood sugar).

“One of the most common problems for older adults are changes in mental status — getting anxious, getting loopy,” said Dr. Sara Cosgrove, medical director of the Johns Hopkins Hospital’s Adult Antimicrobial Stewardship Program. “These drugs get into the brain.” The F.D.A. also warned of the problem in July.

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.

(Fluoroquinolones, for instance, can be useful for hospitalized patients but aren’t first- or even second-line treatments for uncomplicated urinary tract infections; older antibiotics like Bactrim, Septra and Macrobid are.)

Getting this right will mean breaking longstanding habits among providers, including dentists (who may unnecessarily tell patients to take antibiotics before appointments if they’ve had certain surgeries).

Sometimes, though, patients are the ones demanding a pill to end a cold’s miseries, even when there is no such pill.

“We encourage patients not to pressure their physicians to prescribe antibiotics,” Dr. Kabbani advised. And when doctors do prescribe them, “Have a conversation about why. Do I really need it? What else can I do to feel better? What do I watch out for in terms of side effects?”

Caryn Isaacs didn’t do well on Cipro. She suffered severe chest pain — “I thought I was having a heart attack” — and felt anxious and irritable.

She finished the weeklong course of the drug anyway and her personality changes have receded, she said. But occasional twinges of chest pain persist, along with a sense of weakness.

She does worry about urinary tract infections; she has seen the delirium they sometimes cause in her older clients. But she may approach the issue differently next time.

“I probably would take something,” she said. “But I won’t take Cipro.”