Breast cancer is the most commonly diagnosed cancer among women in the United States. Its known risk factors include age, family history, obesity after middle age, and not having children or having a first child after age 35.(101) Antibiotics are crucial in treating a wide variety of infections, and are an extremely commonly prescribed group of medications. Therefore, when media coverage indicated that antibiotic use increases breast cancer risk, the public was understandably alarmed.
Where Did the Scare Come From?
The flurry of media coverage was due to a February 2004 article in the Journal of the American Medical Association finding that prescriptions for antibiotics had been more common among women who had been diagnosed with breast cancer than among women without the disease.(102) The more days women had been prescribed antibiotics, the higher their risk for breast cancer. Among women who had been prescribed antibiotics for 500 or more days over a period of 17 years, the risk of breast cancer was approximately double that of women who had not had any antibiotics prescribed to them. The study looked at the number of days that antibiotics had been prescribed (based on health insurance records), not at the number of days that women actually took antibiotics.
The Media Coverage:
While the study did not conclude that antibiotics were a cause of breast cancer, headlines alerted readers: “Antibiotics may raise risk of breast cancer”(103) and “Women cautioned on use of antibiotics.”(104) The opening line of CNN.com’s coverage stated that the study suggested “increased use of antibiotics may heighten women’s risk of breast cancer.”(105) Some articles did point out that the study didn’t show antibiotic use was a cause of breast cancer, using measured language and indicating that more research was needed before making any conclusions.(106) However, these finer points of science may have gotten lost in much of the press coverage reporting the “link.” As one article recognized, some headlines left a “clear impression” that antibiotics doubled the risk of getting breast cancer, but that “may say more about the danger of first impressions than it does about antibiotics.”(107)
The Bottom Line:
The alarm triggered by the media coverage linking breast cancer and antibiotics stemmed from a lack of understanding about what the research indicated and what it did not. While looking back at women’s medical records, the study found an association between more prescriptions for antibiotics and a higher risk for breast cancer. However, there was no indication that antibiotics themselves elevated breast cancer risk. Furthermore, the potential biological explanation offered by the study’s authors ignored the fact that different antibiotic classes differ in the effects related to the proposed biological mechanism.(108) Instead, both antibiotic use and breast cancer risk may reflect another underlying factor (such as susceptibility to infection or inflammation). Additionally, even aside from the lack of any evidence that antibiotics themselves elevated breast cancer risk, a number of methodological issues hamper confidence in the finding that there was even an association between the two. For example, only 42% of the control group (women not diagnosed with breast cancer) had a mammogram within two years prior to the study. Also, while the women with more than 500 days of antibiotic use recorded had an average of over 20 years of medical records available for inclusion in the study, the women who were recorded as having never used antibiotics had an average of less than 10 years of medical records included. There is no way to know about the antibiotic use of those women during the more than 10 years of missing data.
As the editorial that accompanied the study stated, “this study provides many (or more) questions than answers.”(109) However, while conveying the research to the public, some media obscured the fact that while the study may be helpful as a starting point for future scientific inquiry, it is not a conclusion—or even an indication—about what increases the risk of breast cancer.