Tamoxifen citrate, the non-steroidal anti-estrogen and the first selective estrogen receptor modulator is the treatment of choice for patients with all stages of estrogen receptor positive breast cancer. Although Cytochrome P-450s (CYP) are well known to be responsible for much of its metabolism, fragmentary studies have been carried out to analyze the activity of xanthine oxidase, a non-CYP drug metabolizing enzyme, in tamoxifen metabolism. So, the present investigation is aimed to ascertain the effect of tablet and nano-formulation of tamoxifen on xanthine oxidase expression during 3-methylcholanthrene-induced carcinogenesis. It is found that oral administration of tamoxifen in both the formulations significantly (p < 0.01) decreases the xanthine oxidaseactivity in liver tissue of female mice. We speculate our results to be beneficial for the better understanding of drug metabolism and intensity, which in turn will pave the way for new and efficient nano-formulations of drug coming into therapeutic use.