Women who used increased amounts of antibiotics appear to have a greater risk of breast cancer, according to a new study in The Journal of the American Medical Association (JAMA). However, the researchers point out that more studies are needed to determine if the association between breast cancer and antibiotics is causal or if there are other underlying factors to be considered. "Breast cancer is the most frequently diagnosed nonskin malignancy and the second leading cause of cancer mortality in U.S. women," according to background information from the authors. "It is also the most common cancer in women worldwide, with more than 1 million cases diagnosed each year. Antibiotics are used extensively and overused in many countries, though efforts are underway to curb overuse."
Christine M. Velicer, from the University of Washington, and colleagues, examined the association between use of antibiotics and the risk of breast cancer by reviewing medical data from 10,219 women enrolled at Group Health Cooperative (GHC), a large nonprofit health plan in western Washington State. The study cases included 2,266 women older than 19 years with primary, invasive breast cancer enrolled in the health plan, and 7,953 randomly selected female health plan members who did not have breast cancer in the control group. Data on antibiotic use were obtained from the GHC pharmacy 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 state. For example, compared to 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 one-and-a-half 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 write: "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 article, Roberta B. Ness and Jane A. Cauley from the University of Pittsburgh noted, "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."
