In the study, women who had more than 25 antibiotic prescriptions filled over roughly 17 years had twice the risk of breast cancer as women who never took antibiotics. The risk was smaller, but still elevated, for women who took fewer antibiotics, and the increase in risk was seen for all classes of antibiotics tested.
"We are saying that there is an association between antibiotic use and breast cancer, but we are not saying that antibiotics cause breast cancer," study co-author Stephen H. Taplin, MD, MPH, tells WebMD. "We definitely need to look more closely at this association to try and find the cause."
The National Cancer Institute-supported study is in this week's issue of the Journal of the American Medical Association.
Inflammation Might Explain Link
Taplin says antibiotic use may directly influence breast cancer risk, or the increased risk may be linked to the diseases women are using antibiotics to treat. In an editorial accompanying the study, University of Pittsburgh epidemiologist Roberta Ness, MD, noted that there is compelling evidence linking the inflammation which can result from chronic infection to breast cancer. Such infections are routinely treated with antibiotics.
In an interview with WebMD, Ness said the study should not scare women away from taking antibiotics when they are needed. But she added that there is an urgent need for everyone to take fewer antibiotics when they are not needed.
The over-reliance on antibiotics to treat conditions for which they are not effective, such as colds and other viral infections, has contributed to the worldwide problem of drug-resistant bugs. The U.S. Centers for Disease Control and Prevention (CDC) reports that virtually all significant bacterial infections are becoming resistant to the antibiotic treatment of choice.
"Antibiotics save lives, but the overuse of antibiotics is the thing that scares me most in public health," Ness says. "If the effect of this study is to make a woman think twice about asking her doctor for an antibiotic when she may not need one, that would be a positive thing."
Other Ways to Reduce Risk
In the study, medical records were reviewed for 2,266 breast cancer patients and 7,953 women without cancer enrolled in a Washington State health plan. Lead author Christine M. Velicer, PhD, and colleagues reviewed the total number of antibiotic prescriptions filled and the total number of days on antibiotics.
The researchers found that the more antibiotics the women took over the observation period of roughly two decades, the higher their risk of breast cancer. The study is only the second to examine the impact of antibiotic use on breast cancer. A study from Finland, also involving roughly 10,000 women, also found a link between the two.
American Cancer Society breast cancer spokesperson Debbie Saslow, PhD, agrees that it is far too soon to say that antibiotics cause breast cancer.
"It is important for women who need to be on antibiotics for medically valid reasons to not be afraid to take them," she tells WebMD. "There are many other proven ways to lower breast cancer risk, such as exercising, maintaining body weight, and if you drink alcohol, drink less or stop. And most women have gotten the message about hormone replacement therapy. Limiting the use of HRT [hormone replacement therapy] can also reduce risk."