A pathway for the metabolism of tamoxifen is identified including the activity of at least 2 different isoforms of UGY2B7. This pathway includes isozymes of CYP3A which convert tamoxifen (TAM) to N-desmethyl-TAM, which is then converted to endoxifen by the action of CYP2D6. Once formed, endoxifen may be degraded by at least one isozyme of UGT2B7. Patients that have highly active isoforms of CYP2D6 and slow acting isoforms of UGT2B7 accumulate high levels of endoxifen and are good candidates for treatment with tamoxifen. Also disclosed are methods for screening patients with a high likelihood of benefiting from treatment with tamoxifen. These methods include testing patients for various alleles of genes known to be involved in tamoxifen metabolism and treating patients accordingly.