CHICAGO, March 8 (Xinhua) -- Despite public health campaigns aimed at reducing unnecessary prescriptions for antibiotics, the drugs continue to be prescribed at startlingly high rates in outpatient settings such as clinics and physician offices in the United States, a study of Washington University School of Medicine in St. Louis showed.

The researchers analyzed de-identified data from Express Scripts Holding Co., which manages drug benefits for employers, and found that 98 million outpatient antibiotic prescriptions were filled by 39 million people during a three-year period from 2013 to 2015. And no decline in the overall antibiotic prescription rate is found during the time.

The data tracked monthly prescription rates for all antibiotics, including the five prescribed most often in outpatient settings: azithromycin, amoxicillin, amoxicillin/clavulanate, ciprofloxacin, and cephalexin.

The average number of antibiotic prescriptions per 1,000 beneficiaries was 826 per year. The researchers noted a slight decrease in such rates in 2014, followed by a slight increase in 2015. Overall, the fluctuations were not statistically significant.

The researchers also found seasonal variations in outpatient antibiotic prescriptions. They noted 8,000 to 9,000 antibiotic prescriptions per month in winter compared with fewer than 6,000 antibiotic prescriptions in summer months.

The U.S. Centers for Disease Control and Prevention (CDC) estimated that up to 30 percent of antibiotic prescriptions in outpatient settings may be unnecessary.

"This study suggests that current guidelines on prescribing antibiotics are not being followed," said the study's first author, Michael Durkin, MD, an assistant professor of medicine at the university. "This is concerning because the overuse of antibiotics is costly and contributes to the rise of drug-resistant superbugs."

Another problem resulting from antibiotic overuse is excess health-care costs. In the current study, the researchers calculated the average cost for antibiotics per beneficiary at 23 dollars per year, and it amounts to nearly one billion dollars annually.

"In terms of drug costs alone, the U.S. spends about 9 billion dollars on antibiotics annually," Durkin said. "If 30 percent of prescriptions are unnecessary, this means we're spending about 3 billion dollars on unneeded antibiotics. The medical consequences of antibiotic overuse, including hospitalizations, add to excess health-care costs."

Durkin holds that the study indicates that current guidelines on prescribing antibiotics are not being followed. "If they were, then we would have seen an overall decrease in antibiotic prescribing rates over time."

"Our research group plans to conduct further studies to identify and understand the gap between current antibiotic prescribing practices in the community and clinical practice guidelines," Durkin said.

The study was published March 8 in the journal Infection Control & Hospital Epidemiology.