| Antibiotic use may be associated with breast-cancer risk by affecting immune function and inflammation and by altering intestinal microflora responsible for estrogen and phytochemical metabolism. The objective of this study was to examine the association between antibiotic use and breast cancer. The study design was a nested case-control study. Cases were 2,266 women age 20 and over who were enrolled at Group Health Cooperative (GHC) for at least one year between January 1, 1993 and June 30, 2001 and had primary, invasive breast cancer. Controls were 7,953 randomly selected female members of GHC frequency-matched to cases on age and length of enrollment. Information on type and extent of antibiotic use was obtained from computerized pharmacy records. Increasing number of antibiotic prescriptions and days of use were associated with increased risk of incident and fatal breast cancer, after adjustment for age and length of GHC enrollment. Antibiotic classes associated with the highest risk of incident breast cancer at high levels of use (greater than 100 days) were: macrolides (OR 1.77, 95% CI 1.31–2.39), tetracyclines (OR 1.40, 95% CI 1.14–1.73), and cephalosporins (OR 1.58, 95% CI 1.12–2.25), compared with antibiotic users without prescriptions for these antibiotic classes. In a subgroup of 320 women with high levels of tetracycline or macrolide use, indication for antibiotic use was determined by medical record review. Breast cancer risk was similar among women using antibiotics for acne or rosacea (conditions where severity may be affected by levels of estrogen) and in women using antibiotics for respiratory infections (OR 1.00, 95% CI 0.50–1.98), after adjusting for age and length of enrollment. Antibiotic use may be a breast-cancer risk factor. We cannot completely rule out that the indication for antibiotic use is the pertinent underlying exposure, but our results do not support this possibility. Although further studies are needed, our data support the continued need for prudent use of antibiotics. |
