Bleeding risk in breast cancer patients during concomitant administration of warfarin and tamoxifen: A population-based nested case-control study.

We aimed to investigate whether the concomitant use of tamoxifen with warfarin is associated with higher risk for bleeding among patients with early estrogen-receptor (ER)-positive breast in a population-based nested case-control study. We identified 1787 patients taking warfarin and 92 cases hospitalized for bleeding and found an adjusted odds ratio (OR) of 1.42 (95% confidence interval (CI): 0.84-2.40) for the risk of bleeding in patients treated with warfarin that initiated tamoxifen within the previous 30 days. As a result, we could not definitively rule out a potential association between tamoxifen use during warfarin and bleeding risk in patients with breast cancer.