Observed tamoxifen drug interactions are dependent on both CYP2D6 phenotype and inhibitor potency

Patients on tamoxifen should avoid strong CYP2D6 inhibitors as their endoxifen concentrations are similar to CYP2D6 poor metabolizers. The utility of endoxifen concentrations and which threshold to consider in clinical practice remains unclear. Ultimately, the clinical impact of mild or moderate CYP2D6 inhibitors on CYP2D6 normal or intermediate metabolizer depends on the endoxifen threshold applied.