U.S. doctors may be over-prescribing antibiotics for sinus infections, which are often caused by viruses and not bacteria, according to a study released Monday.

A review of two national surveys of visits to doctors and recommended treatments found antibiotics prescribed for about 82 percent of acute sinus infections and nearly 70 percent of chronic sinus infections, researchers at the University of Nebraska Medical Center in Omaha said.

That "far outweighs the predicted incidence of bacterial causes. The literature repeatedly shows that viruses are by far the most frequent cause of acute rhinosinusitis," the study, published in this week's Archives of Otolaryngology-Head & Neck Surgery, said.

The infections are considered acute when symptoms persist up to a month. They become chronic when they last for three months or more.

Overuse of antibiotics, which are useless against viruses, is causing the evolution of drug-resistant bacteria that must be treated with the most expensive new antibiotics.

But many patients with sinus infections demand an antibiotic, Dr. Hadley Sharp and colleagues said. As many as one-fifth of antibiotic prescriptions for adults are written for a drug to treat sinusitis.

The high level of antibiotic use may partly come from doctors treating secondary infections, Sharp's team said.

"The vast use of these agents makes the statement that they seem to be effective ... or they would have been abandoned," the researchers wrote.

It is also possible that many sinus infections will simply clear up on their own, the researchers added.

"While keeping the goals of treatment in mind, there are concerns about the overuse of antibiotics and the resultant problems, including drug resistance and increasingly virulent bacteria," they wrote.