Puzzling Study Links Antibiotic Use to Breast Cancer

A large study has shown that women who take antibiotics have an increased risk of breast cancer. The study was published in the February 16th issue of the Journal of the American Medical Association (JAMA).

Researchers used computerized pharmacy and breast cancer screening databases to compare antibiotic use of 2,266 women with breast cancer to similar information from 7,953 women without breast cancer. All were age 20 and over, and a wide variety of frequently prescribed antibiotics was examined. The results showed that the more antibiotics women took, the higher their risk of breast cancer. 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 researchers point out that the study does not mean that antibiotics cause breast cancer but only shows an association between the two. Similar results were found in a Finnish study of almost 10,000 women. More studies are required to examine whether specific indications for antibiotic use or times of use would affect the outcome. The authors say that antibiotics can affect bacteria in the intestine and affect how certain foods are broken down that may prevent cancer. Another hypothesis is that the effects of antibiotics on the body's immune response and response to inflammation could be related to the development of cancer. Still another is that underlying conditions leading to the need for antibiotics caused the increased risk, or that a weakened immune system is the cause of the association.

"This trial suggests another piece in the puzzle of factors that may potentially be involved in the development of breast cancer,” said Andrew C. von Eschenbach, M.D., director of the U.S. National Cancer Institute (NCI, Bethesda, MD, USA) and one of the authors. The other authors were from the Group Health Cooperative (GHC), the University of Washington, and the Fred Hutchinson Cancer Center, all in Seattle (WA, USA).