Research links antibiotics, breast cancer

By Warren King
Seattle Times medical reporter

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Women who take even moderate amounts of antibiotics appear to have an increased risk of breast cancer, Seattle researchers have found.

The risk of the disease as much as doubles in those who take the most antibiotics, compared with women who don't take the medications, the scientists concluded in a large study of Group Health Cooperative members.

"A study trend indicated that the more antibiotics taken, the higher the risk for the cancer," said Christine Velicer, an epidemiologist at Group Health's Center for Health Studies and lead author of the research report appearing in tomorrow's edition of the Journal of the American Medical Association.

The study design took into consideration the age of the women; whether they had used hormone-replacement therapy, which has been linked to increased breast-cancer risk, and how long they were enrolled at Group Health.

The scientists don't know why antibiotics might increase the risk of the disease. They emphasized that more research is needed to verify their findings. Factors other than the antibiotics could be involved, they said.

But they also said the research indicated the risk increased with all types of antibiotics, in addition to increasing with the amount of the medications taken.

"All of this suggests that there's something going on with exposure to antibiotics," said John Potter, co-author of the study and director of the Public Health Sciences Division at the Fred Hutchinson Cancer Research Center. Scientists from the University of Washington and the National Cancer Institute also participated in the research.

A limited study in Finland in 2000 found similar results in women who took antibiotics for urinary-tract infections. The Group Health research is the first of its kind in the United States.

Using medical records, the research team compared the antibiotic use of 2,266 breast-cancer patients and 7,953 women who did not have the disease. All of the women were Group Health members for at least one year between Jan. 1, 1993, and June 30, 2001, and were followed for an average of 17 years.

The scientists found that women who took antibiotics for more than 500 days cumulatively, or had more than 25 individual prescriptions, had twice the relative risk of breast cancer as those who didn't take the drugs. Those who had one to 25 prescriptions had about 1½ times the relative risk.
 
If the findings are supported by additional research, Velicer said, the increased risk from antibiotic use will fall in the same range as other known breast-cancer risks, including obesity, hormone use and a family history of the disease.

The scientists speculated that antibiotics affect bacteria in the intestines that might, in turn, affect the metabolism of foods believed to protect against cancer. Or, they said, some drugs could cause inflammation, which has been linked to breast cancer.

Perhaps the antibiotics aren't even the cause of the increased risk, the researchers said: Maybe the women who needed the drugs had weaker immune systems than those who didn't take the medications. Or perhaps the illnesses that required the drugs contributed to development of breast cancer.

"As is often true for reports of new associations, this study provides many, or more, questions than answers. Is the observed link between use of antibiotics and risk of breast cancer confounded by unmeasured factors?" said University of Pittsburgh researchers Roberta Ness and Jane Cauley in an editorial in the Journal of the American Medical Association.

Velicer and her colleagues readily agree more research is needed.

"The next steps in the research need to revolve around these questions," Velicer said. "This study is a spring-off point for other studies."

The scientists said the study is certainly not a basis for making public-health or policy recommendations. But they said it adds fuel to the argument that antibiotics should be used prudently. Experts have warned for years that an increasing number of bacteria are becoming resistant to antibiotics and that the medications should be used sparingly.

Meanwhile, Velicer said, women should talk with their doctors about the risks and benefits of using antibiotics for any given illness. They also should continue following recommendations for screening for breast cancer, she said.

Warren King: 206-464-2247 or wking@seattletimes.com

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