Effects of tamoxifen on blood coagulation.

Sixteen patients with clinically localized breast carcinoma who had been receiving tamoxifen 20 mg twice daily for between 3 and 38 months (median, 14 months) were studied. Several parameters of coagulation (antithrombin III, protein C, fibrinopeptide A and in vitro monocyte procoagulant activity) were investigated in this group and compared to a group of 15 patients with clinically localised breast carcinoma not given tamoxifen. Tamoxifen did not induce significant changes in these parameters to account for the reported thromboembolic events associated with this therapy. The reduced antithrombin III activity previously described in patients receiving tamoxifen for metastatic breast cancer may reflect disease activity rather than a direct effect of tamoxifen on blood coagulation.