Study Shows Link Between Antibiotic Use and Increased Risk of Breast Cancer

Posted: March 1, 2004

A study published February 16, 2004 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; the National Cancer Institute (NCI), a part of the National Institutes of Health in Bethesda, Md.; the University of Washington, Seattle; and the Fred Hutchinson Cancer Center, also in 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 H. Taplin, M.D., of NCI's Division of Cancer Control and Population Sciences and formerly of the GHC. "More studies must be conducted to determine whether there is indeed 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 C. von Eschenbach, M.D.

Study findings
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.

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 similar information from 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.

Possible explanations
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 also possible that the underlying conditions that led to the 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.

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 percent of women had used between one and 25 prescriptions for antibiotics to treat various conditions over an average 17-year period, and only 18 percent of women in the study had not filled any antibiotic prescriptions during their enrollment in the health plan.

"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."

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