Women with a greater cumulative history of antibiotic use appear to have an increased risk for breast cancer, according to an article in the February 18th issue of the Journal of the American Medical Association.

Christine M. Velicer, PhD, and her American colleagues, examined the association between use of antibiotics and the risk of breast cancer by reviewing medical data from 10,219 women enrolled in a large regional health plan. From that total, researchers identified 2,266 women older than 19 years with primary, invasive breast cancer and 7,953 randomly selected women who did not have breast cancer. Data on antibiotic use were obtained from the health plan database. The researchers looked at the cumulative number of days of antibiotic use and the total number of antibiotic prescriptions for each study participant.

"We found that increasing cumulative days of antibiotic use and increasing cumulative number of antibiotic prescriptions were associated with increased risk of incident breast cancer, after controlling for age and length of enrollment," the researchers wrote. For example, compared with women who had used no antibiotics, the researchers found that women who had cumulatively used antibiotics for 1 - 500 days had an associated increased risk of about 1.5- times of incident breast cancer. "Increasing cumulative days of antibiotic use was also associated with death due to breast cancer, controlling for age, length of enrollment, and ever use of postmenopausal hormones. All classes of antibiotics were associated with increased risk."

The researchers also looked at a subset of study participants with heavy use of macrolide and tetracycline antibiotics and found no difference in risk of incident breast cancer among women using these antibiotics for acne and/or rosacea compared with women using these drugs for respiratory tract infections.

The authors offered a few possible explanations for the observed association between antibiotic use and increased breast cancer risk. Antibiotics can affect bacteria in the intestine, which may impact how certain foods that might prevent cancer are broken down in the body. Another hypothesis focuses on antibiotics' effects on the body's immune response and response to inflammation, which could also be related to the development of cancer. It is also possible that the underlying conditions that led to the antibiotic prescriptions caused the increased risk, or that a weakened immune system -- either alone, or in combination with the use of antibiotics -- is the cause of this association.

The authors concluded, "While the implications for clinical practice will not be clear until additional studies are conducted, the results of this study support the continued need for prudent long-term use of antibiotics and the need for further studies of the association between antibiotic use and cancer risk."

In an accompanying editorial, Roberta B. Ness, M.D., M.P.H., and Jane A. Cauley, Dr.P.H., considered the hypothesis that use of prescribed antibiotics may be another risk factor for cancer.

"This observation is potentially worrisome in that antibiotic exposure is common and sometimes nonessential. Thus, if real, the risk of breast cancer attributable to the use of antibiotics could be large and partially preventable," the authors wrote. However, the editorial authors added that the study raises several concerns, including the question of confounding factors in the breast cancer cases cited in the study and the effects of the biological mechanisms of the antibiotics.

"As is often true for reports of new associations, this study provides many (or more) questions than answers…Time and further scrutiny will tell. While more research is needed, this study raises the possibility that long-term use of antibiotics may have harmful consequences, especially for patients for whom other therapeutic options are available."

For additional material in English about this study, please go
to http://www.cancer.gov/newscenter/pressreleases/AntibioticsQandA