Association remains unexplained: antibiotic use linked to increased risk of breast ca; Data 'do not support a change' in practice.

The more antibiotics a woman has taken, the higher her risk of developing breast cancer later and then dying from the disease, results of a large, case-control study suggest.

It's still not clear whether use of antibiotics causes breast cancer, or whether indications for antibiotic use, a weakened system, or other factors could explain the association, study investigators emphasized.

"The results of this study support the continued need for prudent long-term use of antibiotics and the need for further studies of the association between antibiotic use and cancer risk," wrote Christine M. Velicer, Ph.D., of the University of Washington, Seattle, and her associates.

While the study does raise concerns about the potential consequences of long-term antibiotic use, "these findings alone do not support a change in clinical practice. They signal the need for more study," Dr. C. Kent Osborne, director of the Breast Center at Baylor College of Medicine in Houston, said in an interview.

The results offer reason to be mindful of long-term use of antibiotics, "but some patients have conditions requiring antibiotics, and it would not be wise to withhold them thinking about a questionable breast cancer risk down the line," said Dr. Osborne, who was not involved in the study.

The investigators used computerized pharmacy records, some dating back to 1977, to review the histories of antibiotic use for more than 10,000 women enrolled in a health plan. The study included 2,266 women who had received a diagnosis of primary, invasive breast cancer between Jan. 1, 1993, and June 30, 2001, and 7,953 controls matched for age and length of enrollment in the plan (JAMA 291[7]:827-35, 2004).

Only antimicrobial, anti-infective agents were considered. Of the antibiotic prescriptions dispensed to participants, eight classes represented 97% of the total: macrolides, tetracyclines, penicillins, cephalosporins, sulfonamides, nitrofurantoins, metronidazole, and quinolones.