We've known for years that antibiotics don't help in most cases of bronchitis or sore throat. And for decades, public health officials have tried to stop doctors from overprescribing antibiotics.

None of that seems to have made a difference, though: Antibiotics are still being prescribed when they don't help — and could hurt, a study says.

Primary care and emergency room doctors are prescribing antibiotics for a sore throat about 60 percent of the time, according to national health surveys between 1997 and 2010.