New Study Says Antibiotic Use is Linked to Increased Breast Cancer Risk


Newswise -- Antibiotic use is associated with an increased risk of breast cancer, according to a new study conducted by researchers from Group Health Cooperative's Center for Health Studies, the University of Washington, Fred Hutchinson Cancer Research Center, and the National Cancer Institute. The study appears in the Feb. 18, 2004 issue of the Journal of the American Medical Association (JAMA).

"In our study of more than 10,000 women enrolled in Group Health Cooperative over an average of 17 years, we found that the more antibiotics the women took, the more likely they were to develop breast cancer," said Christine Velicer, PhD, a Group Health research associate and the lead author of the study.

The researchers discovered that women who took antibiotics for more than 500 days (or more than 25 individual prescriptions) during their enrollment at Group Health had twice the risk of breast cancer as those who had taken no antibiotics. Women who had between one and 25 prescriptions were about one and a half times more likely to get breast cancer than those who took no antibiotics.

Velicer emphasized that this is the first study of this association in the United States, and more research is needed to evaluate whether it's actually the antibiotics that lead to an increased risk of breast cancer. "It may be that women who take a lot of antibiotics have some other processes happening in their bodies-such as a weak immune response or a hormonal imbalance-which may be an underlying cause of breast cancer, " said Velicer.

She reiterated that the comparison group consisted of women who had never taken antibiotics during their enrollment at Group Health. "Women who have had no antibiotic use may be a uniquely healthy group in terms of general well-being or lifestyle characteristics, and this possibly could account for some-but probably not all-of the increased breast cancer risk we saw among antibiotic users," Velicer said.

The researchers also said it's too early to draw conclusions about the medical and public health implications of the study. "This is not the time to say all antibiotics are bad," said John D. Potter, MD, PhD, director of the Public Health Sciences Division at Fred Hutchinson Cancer Research Center and co-author of the study. "Antibiotics are an extremely useful tool for the treatment of infection, but they must be used appropriately."

At the same time, the study's findings are remarkable in their strength and consistency, said Potter. "We found that the association between breast cancer risk and antibiotic use is similar over the broad range of antibiotics studied," he said. "Also, there's a clear dose response; the women who consumed the greatest amount of antibiotics had the highest risk."

The researchers point to various theories that suggest antibiotics could indeed lead to breast cancer. One theory is that antibiotics affect the bacteria in the intestine, interfering with the metabolism of certain foods that are known protect the body against cancer. Other theories involve antibiotics' impact on the body's immune and inflammatory responses, both of which could be related to the development of cancer. "Much more research is needed in these areas," said Velicer.

The study was conducted using Group Health Cooperative's computerized pharmacy and breast cancer screening databases. The researchers compared differences in antibiotic use among 2,266 women prior to their diagnosis of primary, invasive breast cancer and 7,953 similar women who did not get breast cancer. The study included a wide variety of the most frequently prescribed antibiotic medications. Topical medications and mouth rinses were not included in the study. All the women in the study were age 20 or older.

Until additional research clarifies the association between antibiotic use and breast cancer risk, the researchers offer the following recommendations for women who take antibiotics or who may need them in the future:

· Talk to your doctor about the risks and benefits of antibiotics for your condition.

· Use antibiotics as recommended only when you and your physician have determined that they are the best course for you. Antibiotics are used to treat bacterial infections and should not be used to treat the common cold, which is caused by a virus. (Further guidelines for appropriate use of antibiotics are available from the Centers for Disease Control and Prevention Web site at

· Follow closely your doctor's recommendations regarding breast cancer screening.

The study was funded by grants from the National Cancer Institute and the Gustavus and Louise Pfeiffer Research Foundation.

In addition to Velicer and Potter, the study's authors are Susan R. Heckbert, MD, PhD, of the University of Washington Department of Epidemiology; Johanna W. Lampe, PhD, RD, of Fred Hutchinson Cancer Research Center's Public Health Sciences Division; Carol A. Robertson, RPh, of Group Health Cooperative's Northgate Medical Center; and Stephen H. Taplin, MD, MPH, of the National Cancer Institute and formerly a senior investigator at Group Health Cooperative's Center for Health Studies.

Group Health Cooperative's Center for Health Studies conducts research related to prevention, diagnosis, and treatment of major health problems. Funded primarily through government and private research grants, the Center is located in Seattle, Washington. Group Health is a consumer-governed, nonprofit health care system that coordinates care and coverage. Based in Seattle, Group Health and Group Health Options, Inc. serve more than 540,000 members in Washington and Idaho.

Source: Group Health Cooperative

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