The use of tamoxifen in management of breast cancer is nothing new, but in relatively recent times it has been highlighted that the extent of metabolic activation of the pro-drug form to the active metabolite endoxifen may influence tamoxifen efficacy. Researchers from The Netherlands examined the cases of over 650 pre-menopausal and post-menopausal women with breast cancer who were treated with adjuvant tamoxifen. CYP2D6 genotyping and plasma endoxifen measurements were examined as potential influencers associated with relapse-free survival. No association was found between endoxifen concentrations and relapse-free survival (adjusted hazard ratio, 0.991; 95% CI, 0.946 to 1.038; P = .691), and furthermore no association was found between CYP2D6 genotype and relapse-free survival (adjusted hazard ratio, 0.929; 95% CI, 0.525 to 1.642; P = .799). This study is important because it explores an important real-life application of the much-touted discipline of pharmacogenomics and “personalised medicine.” The result did not pan out the way some advocates of the science might have expected, or indeed hoped for. Read more about the study in the Journal of Clinical Oncology here.