Provided are methods for determining the amount of tamoxifen and its metabolites in a sample by mass spectrometry. In some aspects, the methods provided herein comprise determining the amount of norendoxifen. In some aspects, the methods provided herein comprise determining the amount of norendoxifen and tamoxifen. In some aspects, the methods provided herein comprise determining the amount of norendoxifen and other tamoxifen metabolites. In some aspects, the methods provided herein comprise determining the amount of tamoxifen, norendoxifen, and other tamoxifen metabolites.