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