Pill puzzle: do antibiotics increase breast cancer risk?

After poring over the pharmacy records of more than 10,000 women, researchers have identified a disturbing correlation: Women in the study who had breast cancer tended to have a history of heavier antibiotic use than cancerfree women. Although this study raises the concern that taking microbe-killing drugs increases a woman's risk of breast cancer, the investigators stress that there may be more plausible explanations for the unexpected finding.

"People who are on antibiotics should remain on them if they have a bacterial infection," says Stephen H. Taplin of the National Cancer Institute in Bethesda, Md. "This study is hot saying there's a causal relationship between antibiotics and breast cancer."

The research project originated a few years ago when Taplin was at the University of Washington in Seattle, where he was working with a graduate student, Christine M. Velicer. She was struck by the result of an epidemiological study indicating that Finnish women who had taken antibiotics for urinary tract infections seemed to have an increased risk of breast cancer. That study was limited, however, because it depended on each woman's memory of her antibiotic use, not on medical records.

Working with investigators at the Fred Hutchinson Cancer Research Center and the healthcare provider Group Health Cooperative, both in Seattle, Velicer began a doctoral-dissertation project in which she used nearly 2 decades of pharmacy records to compare the antibiotic use of 2,266 women with breast cancer to that of 7,953 randomly selected women.

The results, which earned Velicer her degree and a publication with her colleagues in the Feb. 18 Journal of the American Medical Association, were startling. Those women who took antibiotics for more than a total of 500 days had around twice the breast cancer risk of women who had taken no antibiotic, the researchers report. Even women who had taken antibiotics for just 1 to 100 days had a modest increase in breast cancer risk. "The risk goes up as the exposure [to antibiotics] goes up" says Taplin.

"Methodologically, it's a very good study, but it needs to be replicated," says Lynn Rosenberg of the Boston University School of Medicine.

In theory, antibiotic use could increase cancer risk by disrupting the natural ecosystem of microbes in the human gut, thereby altering a person's normal physiology. But the antibiotic-cancer correlation could arise without the drugs being to blame. Antibiotic use would also be linked with breast cancer if, for example, infections triggered inflammatory reactions that promoted tumors, says Randall E. Harris of Ohio State University in Columbus.

Or, says Velicer, some women may simply have weak immune systems that leave them susceptible to both breast cancer and infections that require antibiotics.

Velicer and her colleagues plan to investigate whether the use of such medicine is associated with an increased risk of other cancers, such as colorectal cancer.

"This whole issue needs a lot more attention," says Velicer. "It's still a big puzzle."