Tamoxifen is metabolized by cytochrome P450 2D6 (CYP2D6) to the more potent metabolites 4-hydroxy-tamoxifen (4-OH-TAM) and 4-hydroxy-N-desmethyltamoxifen (endoxifen). Variations in the metabolic capacity of this enzyme have shown to be an independent predictor of breast cancer relapse and death. To date, studies have not correlated tamoxifen doses to CYP2D6 genotype status or associated tamoxifen doses to endoxifen and 4-OH-tamoxifen.