In the new study on more than 10,000 women, those who used the most antibiotics — who had more than 25 prescriptions, 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.
"This is an important study, as it appears to be the first major work to describe a possible association between antibiotic use and the increased risk of cancer," said Jeanne Calle, the American Cancer Society's director of analytical epidemiology.
But Calle said additional research is needed to determine if the results are valid.
"We found in association with this study, but association does not necessarily mean cause and effect," cautioned breast cancer surgeon Elisa Rush Port of the Memorial Sloan-Kettering Cancer Center in New York on CBS News' The Early Show.
The authors agreed and said it could have been the diseases women used antibiotics to treat — rather than the drugs themselves — that increased breast cancer risk.
Also, since antibiotics are widely used to treat a variety of common infections caused by bacteria, including strep throat, 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.
"It's very premature for people to stop taking antibiotics when they're needed," said lead author Christine Velicer, a researcher at Group Health Cooperative, a large nonprofit health plan in western Washington. "Antibiotics have a substantial public health benefit."
But the drugs, which treat only bacterial ailments, are often inappropriately used for colds and other illnesses caused by viruses, and the authors said women should use antibiotics only when they've discussed with their doctors whether the drugs are the most appropriate treatment for their ailments.
"There are public health issues in terms of taking too much antibiotics, developing drug-resistant bacteria and so forth," said Dr. Port. "It's always been a concern public health-wise. This is just one more reason why we need to be careful."
The results appear in Wednesday's Journal of the American Medical Association and were released Monday.
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 — a breast cancer rate about 1.5 times higher than nonusers.
An increased breast cancer risk was found for all types of antibiotics, including penicillins.
A few theories might explain how antibiotics would lead to breast cancer, Velicer said. The medications' effects on intestinal bacteria could change the body's immune system or how the body metabolizes foods that protect against cancer, she said.
An JAMA editorial says the theorized link is potentially worrisome, since antibiotics are so commonly used — and misused.
But more research is needed before the drugs can be implicated, said editorial authors Dr. Roberta Ness and Jane Cauley of the University of Pittsburgh.