As if increasing rates of antimicrobial resistance were not enough to discourage excessive use of antimicrobial agents, now it seems that antibiotic use is associated with an increased risk of breast cancer.
Researchers conducted a casecontrol study of 2,266 women older than 19 years with primary invasive breast cancer. They found that the risk of incident and fatal breast cancer increased as did the total number of days of antibiotic use, from an odds ratio for incidence of 1.45 for 1 to 50 days of use to an odds ratio of 2.07 for greater than 1,000 days of use (P<0.001). Increased risk was associated with all antibiotic classes, but not with tetracycline or macrolides used strictly for acne or rosacea.
Researchers are uncertain whether the antibiotics themselves increase the risk of breast carcinoma or whether a weakened immune system predisposes certain women to both an increased number of infections requiring antibiotic treatment and to breast cancer.
Velicer CM, Heckbert SR, Lampe JW, et al. Antibiotic use in relation to the risk of breast cancer. JAMA. 2004;291:827-835.