US doctors prescribe antibiotics to six out of 10 patients with a sore throat, even though only one infection in 10 is severe enough to merit them, researchers said Thursday.

Overprescribing of antibiotics is dangerous because it contributes to the rise of superbugs that do not respond to treatment.

US health officials have repeatedly warned that nearly all major bacterial infections in the world are becoming resistant to the antibiotic treatment of choice.

The latest study by Michael Barnett and Jeffrey Linder at Harvard University and Brigham and Women's Hospital included data on more than 8,100 visits to doctors' offices and emergency departments from 1997-2010.

Antibiotic prescribing rates were initially around 80 percent and have bottomed out at 60 percent for the past decade, said the research letter in the Journal of the American Medical Association.

"Among adults seeking care with sore throat, the prevalence of group A Streptococcus (GAS) infection -- the only common cause of sore throat requiring antibiotics -- is about 10 percent," said the letter.

The research shows that despite concerns about rising resistance to antibiotics, physicians are not changing their habit of regularly prescribing remedies like penicillin, amoxicillin and erythromycin which may not be necessary.

"Despite decades of effort, we found only incremental improvement in antibiotic prescribing for adults making a visit with sore throat," said the letter in JAMA.

"The antibiotic prescribing rate dropped from roughly 80 percent to 70 percent around 1993 and dropped again around 2000 to 60 percent, where it has remained stable."

Researchers noted that their data did not include details on the severity of the sore throats, so it was impossible to judge whether the prescriptions given were appropriate.

"Antibiotic prescribing to patients who are unlikely to benefit is not benign. All antibiotic prescribing increases the prevalence of antibiotic-resistant bacteria," the doctors concluded.

"The financial cost of unnecessary antibiotic prescribing to adults with sore throat in the United States from 1997 to 2010 was conservatively $500 million."