Results of a Finnish cohort study showed an association between antibiotic use and breast cancer risk in women younger than 50 years. Given the high prevalence of breast cancer and the widespread use of antibiotics, Velicer and colleagues performed a case-control study to determine whether an association exists between breast cancer risk and antibiotic use.

Patients with newly diagnosed primary breast cancer were identified, along with matched control patients. Data on antibiotic use were obtained from pharmacy records. Two measures of antibiotic exposure were employed: the cumulative number of days of antibiotic use and the total number of antibiotic prescriptions for each study participant. Antibiotic use was categorized as low, moderate, or high. Detailed information about risk factors for breast cancer was available for study enrollees. In a substudy, a chart review allowed investigators to ascertain the indications for antibiotic use.

For all eight antibiotic classes considered, increasing cumulative days of use were associated with increased risk of incident breast cancer. For categories of increasing days of use (i.e., zero, one to 50, 51 to 100, 101 to 500, 501 to 1,000, and 1,001 or more days), the odds ratios for breast cancer were 1.00, 1.45, 1.53, 1.68, 2.14, and 2.07. Similar results were found when the number of antibiotic prescriptions was the focus of the analysis. In addition, a strong association was noted between antibiotic use and death from breast cancer. None of these results was affected when adjusting for demographic and breast cancer risk variables.

The authors conclude that there is an association between increasing antibiotic use as measured by cumulative number of days and prescriptions, and increased risk of incident breast cancer. The findings apply to pre- and post-menopausal women. In a sub-analysis, indication for antibiotic use was not associated with breast cancer risk, but because of the small size of the subset, indication may play a role. The study, which controlled for many risk factors, could not ascertain whether the association of antibiotic use and cancer risk is causal, or whether antibiotic use is related to some other cause (such as a weakened immune system) or medical indication.