LAUSR

Laboratory studies suggest that inhibition of the cyclooxygenase (COX)‐2 enzymes inhibits breast cancer development. We aimed to evaluate whether postdiagnosis use of COX‐2 selective or other nonaspirin nonsteroidal anti‐inflammatory drugs (NSAIDs) reduce the risk of contralateral breast cancer (CBC) among Danish breast cancer patients. From the clinical database of the Danish Breast Cancer Group, we identified 52,723 women diagnosed with breast cancer between 1996 and 2012. Data on nonaspirin NSAID use, CBC and potential confounding variables were obtained from nationwide registries. We defined postdiagnosis use (two or more prescriptions) as a time‐varying covariate with a one‐year lag. Cox proportional hazard regression models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for CBC associated with nonaspirin NSAID use. During a median follow‐up of 4.8 years (interquartile range: 2.3–9 years), 1,444 patients were diagnosed with CBC. Overall, postdiagnosis use of nonaspirin NSAID was associated with an adjusted HR for CBC of 0.98 (95% CI: 0.87–1.11). The HRs did not vary substantially with duration or intensity of nonaspirin NSAID use. Moreover, similar associations were found for COX‐2 selective (HR: 1.02; 95% CI: 0.85–1.23) and nonselective (HR: 0.96; 95% CI: 0.82–1.13) nonaspirin NSAIDs. In conclusion, our nationwide cohort study of breast cancer patients does not suggest a reduced risk of CBC with nonaspirin NSAID use regardless of the COX‐2 selectivity.