Study Links Breast Cancer to Antibiotics Use

Velicer, Taplin and their colleagues examined computerized pharmacy and cancer screening records of 2,266 women in the Group Health Cooperative, a Seattle-area health plan, who developed breast cancer, and 7,953 similar women who did not get breast cancer.

Women who had more than 25 individual prescriptions for antibiotics over an average period of 17 years had twice the risk of breast cancer as those who had taken no antibiotics. The risk was lower for women who took fewer antibiotics, but even those who had between one and 25 prescriptions were about 1.5 times more likely to get breast cancer, the researchers found.

"It was surprising for me that there was an association," said Velicer. "The overall robustness and the consistency across a number of common antibiotics was really notable."

Antibiotics are regularly prescribed for a wide variety of conditions, such as respiratory infections, acne, and urinary tract infections. In the study, more than 70 percent of women had used between one and 25 prescriptions for antibiotics, and only 18 percent had not filled any antibiotic prescriptions.

The researchers tried to explain the results by looking at other known risk factors, such as a family history of breast cancer, hormone use and mammograms. But none did. They also did a separate analysis comparing women who were taking large amounts of antibiotics because of a skin condition associated with a hormonal imbalance with those taking antibiotics because of respiratory infections so see if the real cause might be the hormonal imbalance. That too failed to explain the findings, though it could not completely rule it out.

One finding that cast doubt on the possibility that it will turn out that antibiotics increase the risk for breast cancer was that the study found the risk for all types of antibiotics, said Debbie Saslow, director of breast and gynecologic cancer for the American Cancer Society. That makes it unlikely it's the antibiotics because they different classes work in very different ways, she said.

"There are a lot of things associated with cancer risk that have absolutely nothing to do with causing cancer," she said.

Jeanne Calle, the society's director of analytical epidemiology, called the study "important" because "it appears to be the first major work to describe a possible association between antibiotic use and the increased risk of cancer.

But "it is critical to realize we cannot say with any certainty how valid these results are until and unless they are replicated in additional studies," she said. "There is no question other researchers will now begin to investigate this potential association. Such additional studies will clarify the role, if any, of antibiotic use and breast cancer risk."