Nearly a third of antibiotics prescribed in U.S. doctors’ offices, emergency rooms and hospital-based clinics are not needed, according to the most in-depth study yet to examine the use and misuse of these lifesaving drugs.

The finding, which has implications for antibiotics’ diminished efficacy, translates to about 47 million unnecessary prescriptions given out each year across the country to children and adults. Most of them are for conditions that don’t respond to antibiotics, such as colds, sore throats, bronchitis, flu and other viral illnesses.

Health officials have been warning for decades about the overuse of antibiotics and its contribution to the development of drug-resistant bacteria, but the research from the Centers for Disease Control and Prevention and the Pew Charitable Trust is the first to quantify the depth of the problem.

The study, published Tuesday in JAMA, analyzed data for all antibiotic use in the three settings as collected from two major CDC surveys from 2010 to 2011. Antibiotic prescriptions written there represent the majority of dollars spent on antibiotics in U.S. health care.

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The study found that about 154 million, or 13 percent, of all U.S. outpatient visits annually result in an antibiotic prescription.

Doctors often prescribe them because of pressure from patients or parents, said Katherine Fleming-Dutra, a CDC medical epidemiologist and the report’s lead author.

An editorial in JAMA noted the numbers likely is an undercount because they don’t include the times antibiotic prescriptions are given over the phone, at urgent care clinics, retail pharmacies and dental offices, or written by nurse practitioners and physician assistants.