A study published in the Journal of the American Medical Association (JAMA) provides evidence that use of antibiotics is associated with an increased risk of breast cancer. The authors, from Group Health Cooperative (GHC) in Seattle; National Cancer Institute (NCI), a part of National Institutes of Health in Maryland; University of Washington, Seattle; and the Fred Hutchinson Cancer Center, Seattle, concluded that the more antibiotics the women in the study used, the higher their risk of breast cancer.

The results of this study do not mean that antibiotics cause breast cancer. “These results only show that there is an association between the two,” explained co-author Stephen Taplin, of NCI’s Division of Cancer Control and Population Sciences and formerly of the GHC. “More studies must be conducted to determine if there is a direct cause-and-effect relationship.”

“This trial suggests another piece in the puzzle of factors that may potentially be involved in the development of breast cancer,” said NCI director Andrew von Eschenbach. “The NCI will continue to support research into underlying mechanisms of cancer risk.”

The authors of this JAMA study found that 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. The risk was smaller for women who took antibiotics for fewer days. However, even women who had between one and 25 prescriptions over an average period of 17 years had an increased risk; they were about 1.5 times more likely to be diagnosed with breast cancer than women who didn’t take any antibiotics. The authors found an increased risk in all classes of antibiotics that they studied.

“Breast cancer is the second leading cause of cancer deaths among women in the US, with an estimated 40,000 deaths this year, and is the most common cancer in women worldwide,” said first author Christine Velicer, of GHC’s Center for Health Studies. “Antibiotics are used extensively in this country and in many parts of the world. The possible association between breast cancer and antibiotic use was important to examine.”

To gather the necessary data, the researchers used computerized pharmacy and breast cancer screening databases at GHC, a large, non-profit health plan in Washington state. They compared the antibiotic use of 2,266 women with breast cancer to 7,953 women without breast cancer. All the women in the study were age 20 and older, and the researchers examined a wide variety of the most frequently prescribed antibiotic medications.

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. It is possible that the underlying conditions that led to antibiotics prescriptions caused the increased risk, or that a weakened immune system, either alone, or in combination with the use of antibiotics, is the cause of this association.

The results of the study are consistent with an earlier Finnish study of almost 10,000 women. “Further studies must be conducted, though, for us to know why we see this increased risk and the full implications of these findings,” said Velicer. Studies are also necessary to clarify whether specific indications for antibiotic use, such as respiratory infection or urinary tract infection, or times of use, such as adolescence, pregnancy or menopause, are associated with increased breast cancer risk. Additionally, breast cancer risks could differ between women who take low-dose antibiotics for a long period of time and women who take high-dose antibiotics only once in a while.

Antibiotics are regularly prescribed for conditions such as respiratory infections, acne, and urinary tract infections, in addition to a wide range of other conditions or illnesses. In this JAMA study, for example, more than 70 per cent of women had used between one and 25 prescriptions for antibiotics to treat various conditions over an average 17-year period, and only 18 per cent of women in the study had not filled any antibiotic prescriptions during their enrollment in the health plan.

Over the past decade, overuse of antibiotics has become a serious problem. According to the Centers for Disease Control and Prevention (CDC), tens of millions of antibiotics are prescribed for viral infections that are not treatable with antibiotics, contributing to the troubling growth of antibiotic resistance. Efforts are underway such as the “Get Smart: Know When Antibiotics Work” campaign — unveiled last year by the Department of Health and Human Services’ CDC and the Food and Drug Administration (FDA) and other partners — to lower the rate of antibiotic overuse.

“These study results do not mean that women should stop using antibiotics to treat bacterial infections,” stressed Taplin. “Until we understand more about the association between antibiotics and cancer, people should take into account the substantial benefits that antibiotics can have, but should continue to use these medicines wisely.”