TAMOXIFEN AND WARFARIN AND OTHER ORAL ANTICOAGULANTS

The anticoagulant effects of acenocoumarol and warfarin are markedly increased by tamoxifen and bleeding has been seen.
Monitor the effects closely if tamoxifen is added to treatment with warfarin or acenocoumarol and reduce the dosage as necessary. Reports indicate that a reduction of between one-half to two-thirds may be needed for warfarin but some patients may need much larger reductions. The dosage reduction needed for acenocoumarol is not known. In the US, when tamoxifen is being used for the primary prevention of breast cancer, the manufacturer specifically contraindicates concurrent use with warfarin.