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To the Editor: Dr Velicer and colleagues1 reported an association between use of antibiotics and increased risk of breast cancer. We have several concerns about the design of this study. First, because the authors did not obtain data about other prescription medications, they could not establish that the effect was specifically due to antibiotics. Second, although the regression analyses were adjusted for age and duration of insurance enrollment, the authors apparently did not adjust for other documented confounders, such as body mass index, family history of breast cancer, and age at menarche. Third, other confounding factors that might be correlated with both the need for antibiotics and the risk of breast cancer were not analyzed (eg, smoke exposure, weight fluctuations, and use of ethanol). Finally, the results are inconsistent with data from animal studies that show no relationship between extended antibiotic exposure and breast cancer.2 Until these issues are addressed we believe that the judicious use of antibiotics is still safe in appropriate patients regardless of the patient's individual risk of breast cancer.

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