CYP2D6 and breast cancer: However, in a randomised trial of tamoxifen 40 mg vs 20 mg in women with advanced breast cancer and poor metaboliser CYP2D6 genotypes no difference in PFS rates at 6 months was seen (67.6% vs 66.7%, respectively) despite significantly higher Z-endoxifen levels in the high-dose arm (median, 89.2 nM v 51.1 nM; P < 0.0001) [52].