Women who took the most antibiotics , or who took the drugs for at least 501 days -- faced double the risk of developing breast cancer over an average of about 17 years, compared with women who didn't use the drugs, the study showed.
Also, since antibiotics are widely used to treat a variety of common infections caused by bacteria, some pneumonias and many gastrointestinal infections, it may be that women who never took the drugs were unusually healthy and therefore unusually resistant to cancer, the researchers said.
The study involved 2,266 women 20 and older who developed invasive breast cancer and who were compared with 7,953 women who did not get breast cancer. An increased breast cancer risk was found with increasing use of antibiotics, with the greatest increased risk in women who took the drugs for at least 501 days. Even women who had up to 25 prescriptions over about 17 years faced an increased risk -- about 1.5 times higher than nonusers. An increased breast cancer risk was found for all types of antibiotics, including penicillins.
Researchers suggest a few theories that might explain how antibiotics would lead to breast cancer, the medications' effects on intestinal bacteria could change the body's immune system or how the body metabolizes foods that protect against cancer, they said.
Specialists says the theorized link is potentially worrisome, since antibiotics are so commonly used -- sometimes unnecessarily for conditions in which they are ineffective, including colds and other viruses. But researchers say more research needed because it could have been the diseases women used antibiotics to treat -- rather than the drugs themselves -- that increased breast cancer risk.