Do antibiotics cause breast cancer?

The Short Answer:

Probably not.

The Long Answer:

In early 2004, a study linking antibiotics to breast cancer splashed onto the scene. What is the nature of this link? The study indicated higher prevalence of antibiotic use among 2226 women with breast cancer compared to 7953 without. Still, these data do not indicate a causal link between antibiotics and breast cancer. In other words, we still do not know if antibiotic use causes breast cancer. The reason for the ambiguity is in the probability of confounding factors. Because a randomized study of this nature would be unethical, the researchers chose to analyze existing data.  Such case-control studies are susceptible to confounding factors, since researchers cannot control all of the variables involved, such as the distribution of antibiotics and the distribution of existing medical conditions among the groups.

For example, perhaps women who use antibiotics are intrinsically less healthy than those who don't, and their breast cancer comes from other sources. Another possible counfounding issue is the possibility that antibiotics actually help prevent breast cancer, and that prevalence of breast cancer would be much higher in this sample population had these women not used antibiotics.

Socioeconomic influences may also cause statistical problems for the validity of the study. Women who have more economic resources may receive more antibiotic use as well as more preventative care for breast cancer. This increase in preventive care may explain the increase in breast cancer detection among those who used antibiotics. In other words, perhaps breast cancer occurs just as frequently in the control group, however it goes undetected and therefore is not counted in the study, In fact, only 42% of the control group received mammographies in the two years preceding the study.

The authors of the study do claim to statistically adjust for most of the confounding possibilities, however critics point out that too many missing variables exist to conclude a statistical link. Moreover, even if a correlation exists, biologists are still unsure of a biological mechanism. Two theories about how antibiotics could chemically cause breast cancer do exist, although neither have been verified.

The Phytochemical Scenario

Antibiotics might impair the body's ability to metabolize phytochemicals. These phytocemicals help break down carcinogens, which may go on to cause breast cancer.

The Cyclooxygenase 2 Scenario

Cyclooxygenase 2 is an enzyme that has been previously associate with breast cancer. The body uses cyclooxygenase 2 to break down prostaglandin E2. Antibiotics might stimulate the creation of prostaglandin E2 in the body, which would cause the body to over produce cyclooxygenase 2.


Velicer CM, Heckbert SR, Lampe JW, Potter JD, Robertson CA, Taplin SH. Antibiotic use in relation to the risk of breast cancer. JAMA. 2004;291:827-835.

Ness RB, Cauley JA. Antibiotics and Breast Cancer- What's the meaning of this? JAMA. 2004;291:880,881