However, the investigators add that although a relationship has
been found, their findings do not prove that antibiotic use is the
cause of breast cancer in these women and they note that other
factors may be involved.
Earlier reports have suggested a link between antibiotics and
increased cancer risk, lead author Dr. Christine M. Velicer of the
University of Washington, Seattle and colleagues note.
The effects of these drugs on the microorganisms normally found
in the intestines and the way estrogen is processed, as well as
their effects on the immune system and on inflammation, have been
suggested as possible ways antibiotics may influence the development
To further investigate a possible relationship, Velicer's group
evaluated data collected for 2266 women with primary, invasive
breast cancer between 1993 and 2001, and 7953 randomly selected
women without breast cancer who were enrolled in the same health
"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," they report.
When compared with women with no history of antibiotic use, women
with cumulative antibiotic use for 1 to 500 days had a 2.5-fold
increase risk of 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," they write.
The investigators stress, however, that "it cannot be determined
from this study whether antibiotic use is causally related to breast
cancer, or whether indication for use, overall weakened immune
function, or other factors are pertinent underlying exposures."
"This observation is potentially worrisome in that antibiotic
exposure is common and sometimes nonessential," Drs. Roberta B. Ness
and Jane A. Cauley of the University of Pittsburgh comment in an
accompanying editorial. "Thus, if real, the risk of breast cancer
attributable to the use of antibiotics could be large and partially
The study provides more questions than answers, they add.
Both the authors and editorialists agree that the clinical
implications of the study are unclear and than more studies are
needed to explore the relationship between antibiotic use and cancer
SOURCE: Journal of the American Medical Association, February 18,