Antibiotics and the risk of breast cancer

Two recent studies found a positive association between antibiotic use and the risk of breast cancer.

Using information from the U.K. General Practice Research Database, we identified 1268 cases of incident breast cancer (40- to 79-year-old women diagnosed in 1987 to 2002 who had at least 6 years of history recorded in the General Practice Research Database) and 6291 female controls matched to the cases on age, general practice, and duration of history recorded in the General Practice Research Database. We ascertained antibiotic prescriptions (penicillins, cephalosporins, sulfonamides, macrolides, tetracyclines, quinolones, metronidazole, and nitrofurantoin) recorded up to 1 year before the index date.

Odds ratios (95% confidence intervals) of breast cancer for 0, 1-50, 51-100, 101-500, and 501 or more cumulative days of antibiotic use were 1.0 (reference), 1.0 (0.9-1.2), 0.9 (0.7-1.2), 0.9 (0.7-1.3), and 1.2 (0.6-2.4).

These data do not support the hypothesis that antibiotic use is associated with an increased risk of breast cancer.