Filed at 4:23 p.m. ET

CHICAGO (AP) -- Doctors may be inflating the costs of treating urinary tract infections -- and possibly promoting resistance to antibiotics -- by ignoring treatment guidelines, according to a new study.

The study, published in Monday's Archives of Internal Medicine, suggests that doctors are driven by drug company promotions to use newer, more expensive drugs.

``If every drug can work and one drug is promoted more heavily, doctors tend to prescribe the one they've heard more about,'' said co-author Dr. Elbert Huang, of the University of Chicago.

Researchers from the University of Chicago and Stanford University studied 1,478 cases of patients with urinary tract infections nationwide from 1989 to 1998.

They found that only 24 percent of the patients' prescriptions were written for antibiotics recommended by the Infectious Disease Society of America, down from 48 percent a decade earlier.

Huang said some of the drugs being prescribed are many times more expensive than the recommended medications.

The guidelines call for using trimethoprim-sulphamethoxazole, which costs $1.79 for a 10-day course. But 30 percent of the cases studied involved prescriptions of nitrofurantoins, such as Macrodantin, which cost $20.34 for the same period. Doctors prescribed a third class, fluoroquinolones such as Cipro, to 29 percent of patients, at a cost of $70.98 for 10 days.

Huang said there also is a concern -- though ``it's very theoretical'' -- that having several classes of antibiotics in use at the same time might cause infections to become resistant to all those classes at once.

He said the guidelines were established with the idea that the recommended drugs could be used until they were ineffective, and that another class would then take their place.

But other physicians say the risk of resistance is unclear.

Dr. Thomas Hooton, an infectious disease specialist at the University of Washington in Seattle who was not involved in the study, said he doesn't fault doctors for choosing alternatives.

``I do it myself,'' said Hooton. ``It really depends on one's point of view. Some say, 'Resistance (to trimethoprim-sulphamethoxazole) is increasing, and I want to use the antibiotic that is most likely to succeed.' ``

He said West Coast doctors are especially justified in prescribing alternatives because one-third of urinary tract infections there are resistant to the recommended antibiotic. That compares to just 7 percent on the East Coast and 14 percent in the Midwest.

``The debate is, what level of resistance is too high and when do you go to the other antibiotic?'' Hooton said.