Women who took antibiotics for more than 500 days--or had more than 25 prescriptions--over an average period of 17 years had more than twice the risk of breast cancer as women who had not taken any antibiotics. Even women who had between one and 25 prescriptions over an average period of 17 years were about 1.5 times more likely to be diagnosed with breast cancer than women who didn't take any antibiotics. The authors offer a few possible explanations for the observed association between antibiotic use and increased breast cancer risk. Antibiotics can affect bacteria in the intestine, which may impact how certain foods that might prevent cancer are broken down in the body. Another hypothesis focuses on antibiotics' effects on the body's immune response and response to inflammation, which could also be related to the development of cancer.
Science Daily February 17, 2004