Medical researchers concluded years ago that antibiotics don't cure acute bronchitis. The drugs might reduce your coughing symptoms slightly, but the antibiotics also increase your risk of side effects such as diarrhea and nausea. And overuse of antibiotics breeds bacteria resistant to the current cures.

Yet patients who show up to clinics or emergency rooms with bronchitis still receive antibiotics about 70 percent of the time, according to a new review of prescribing data published in JAMA: The Journal of the American Medical Association. And the rate even increased slightly between 1996 and 2010 despite efforts to educate doctors and establish guidelines against antibiotics.

"You're taking a medicine that has no chance of helping you and has a very real chance of harming you," says Dr. Jeffrey Linder of Brigham and Women's Hospital in Boston, co-author of the research note. Beyond unpleasant side effects, antibiotics can increase risk of a colon infection called C. difficile and other more serious complications.

The study shows how far evidence-based medicine has to go. The idea that doctors' decisions should be guided by rigorous trials into the effectiveness and safety of different therapies is a big part of what distinguishes modern medical care from earlier eras of bloodletting and leeches. All the sophisticated medical technology and pharmaceuticals developed in the past century won't help patients if they get the wrong treatment at the wrong time.

The Centers for Disease Control joined with medical societies in 2001 to recommend against prescribing antibiotics for acute bronchitis, a respiratory infection that comes with a nasty cough. The evidence against the practice is so clear that health-care providers are measured on how well they avoid it in quality ratings used by insurance companies and the government to evaluate effective care.

The JAMA review excluded patients with other conditions that might warrant antibiotics. It also excluded the elderly and children to get as clear a picture as possible of cases of acute bronchitis in otherwise healthy people, Linder says. The study has some limitations; excluding so many patients left a relatively small sample size. Still, the researchers were looking at a situation where the prescribing rate "should be zero," Linder notes, and found that it was 71 percent.

Linder says the guidelines for bronchitis are widely understood by doctors. Patient demand—or, in some cases, doctors' assumptions that people want antibiotics—leads them to write prescriptions anyway. His advice: "If you develop a cough and don't have a fever and otherwise feel pretty well, the normal duration of cough is three weeks, and taking an antibiotic doesn't shorten that."