Study Links Breast Cancer to Antibiotics Use

Antibiotic use is associated with an increased risk for breast cancer, a new study has found, raising the possibility that women who take the widely used medicines are prone to one of the most feared malignancies.

The first-of-its-kind study of more than 10,000 Washington state women concluded that women who used the most antibiotics had double the chances of being struck by breast cancer, that the association was consistent for all forms of antibiotics and that the risk went up with the number of prescriptions, a powerful indication that the link was real.

A variety of experts quickly cautioned, however, that the findings should not stop women from taking the often life-saving drugs when they need them to treat infections. There could be other explanations for the association, and much more research is needed before scientists understood what the surprising results mean, they said.

"This is not saying that women should stop taking antibiotics. Women should take antibiotics for infections," said Stephen H. Taplin, a senior scientist at the National Cancer Institute who helped conduct the study. "We need to follow-up and find out if this a real association."

Nevertheless, the study was so well designed and the findings were so striking that it could be that antibiotic use is an important, previously unrecognized risk factor for breast cancer, experts said.

Antibiotics could increase the risk for breast cancer by, for example, affecting bacteria in the digestive system in ways that interfere with the metabolism of foods that protect against cancer, experts said. Another possibility is that antibiotics boost the risk by affecting the immune system or the body's inflammatory response.

Even if it turns out antibiotics do not increase the risk for breast cancer, the finding is likely to turn out to be important because it could lead to the discovery of whatever it is about women who use the drugs that also appears to make them prone to the disease, researchers said. "This has opened up a picture that people had not been thinking about," Taplin said. "The important thing is more research and asking more questions about what it could be."

Until the results are sorted out, experts said, the findings provide yet another reason for doctors to more judiciously prescribe antibiotics, which are often used unnecessarily, especially for women who may be at risk for breast cancer for other reasons.

"It's a very provocative finding but it's not entirely clear what it means," Roberta B. Ness, an epidemiologist at the University of Pittsburgh, who co-authored an editorial accompanying the study in this week's Journal of the American Medical Association. "The first thing you have to ask is if it's real. I think a cautious interpretation is very reasonable."

The researchers took pains to try to find other explanations for the association, such as the possibility that women who take antibiotics are more likely to be diagnosed with breast cancer because they see doctors more often. But the association remained even after they excluded that and all of the other most likely possibilities.

The study's design, however, made it impossible to rule out the chance that women who tend to use the medicines are somehow biologically predisposed to breast cancer for other reasons, such as by having weak immune systems or a hormonal imbalance that is the real cause of both their increased risk for breast cancer and for infections that prompted antibiotic use.

Scientists first proposed that antibiotics may increase the risk for breast cancer in 1981, but the only other study to examine the question was in Finland in 2000. That study also found an association, but it was not as well designed, prompting the new research.

"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," said Christine M. Velicer, an epidemiologist with Group Health Cooperative's Center for Health Studies who was the lead author of the study.

Breast cancer strikes more than 211,000 women each year in the United States and kills more than 40,000, making it the leading cause of cancer and second leading cancer killer among women.