Tamoxifen Coumarins

Evidence is limited to the above reports, but it appears that a clinically important interaction between tamoxifen and warfarin can occur, which apparently affects some but not all patients. Monitor the effects closely if tamoxifen is added to treatment with warfarin or acenocoumarol and reduce the dosage as necessary. The reports cited here indicate a reduction of between one-half to two-thirds for warfarin. In the US, when tamoxifen is being used for the primary prevention of breast cancer, the manufacturer specifically contraindicates concurrent use with warfarin or other coumarins. ... 

Because of the data with tamoxifen, the manufacturers of toremifene (indicated for hormone-dependent metastatic breast cancer in postmenopausal women) contraindicate the concurrent use of coumarin anticoagulants in the UK, but just recommend careful monitoring of prothrombin time in the US. However, there appear to be no published reports of any interaction between toremifene and warfarin. 

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