Tamoxifen drug interactions

List of drug interactions
Coumarin-type anticoagulants Cytotoxic agents Antineoplastic drugs Phenobarbital Rifampin Aminoglutethimide Medroxyprogesterone Bromocriptine therapy

Coumarin-type anticoagulants
When Tamoxifen is used in combination with coumarin-type anticoagulants, a significant increase in anticoagulant effect may occur. Where such coadministration exists, careful monitoring of the patient's prothrombin time is recommended. In the NSABP P-1 trial, women who required coumarin-type anticoagulants for any reason were ineligible for participation in the trial. Return to top

Cytotoxic agents
There is an increased risk of thromboembolic events occurring when cytotoxic agents are used in combination with Tamoxifen. Return to top

Antineoplastic drugs
Tamoxifen reduced letrozole plasma concentrations by 37%. The effect of Tamoxifen on metabolism and excretion of other antineoplastic drugs, such as cyclophosphamide and other drugs that require mixed function oxidases for activation, is not known. Tamoxifen and N-desmethyl Tamoxifen plasma concentrations have been shown to be reduced when coadministered with rifampin or aminoglutethimide. Induction of CYP3A4-mediated metabolism is considered to be the mechanism by which these reductions occur; other CYP3A4 inducing agents have not been studied to confirm this effect. Return to top

Phenobarbital
One patient receiving Tamoxifen with concomitant phenobarbital exhibited a steady state serum level of Tamoxifen lower than that observed for other patients (ie, 26 ng/mL vs. mean value of 122 ng/mL). However, the clinical significance of this finding is not known. Return to top

Rifampin
Rifampin induced the metabolism of Tamoxifen and significantly reduced the plasma concentrations of Tamoxifen in 10 patients. Return to top

Aminoglutethimide
Aminoglutethimide reduces Tamoxifen and N-desmethyl Tamoxifen plasma concentrations. Return to top

Medroxyprogesterone
Medroxyprogesterone reduces plasma concentrations of N-desmethyl, but not Tamoxifen. Return to top

Bromocriptine therapy
Concomitant bromocriptine therapy has been shown to elevate serum Tamoxifen and N-desmethyl Tamoxifen. Return to top