CYP2D6 and breast carcinoma: In spite of genetic variations among patients, it has been shown that venlafaxine is a weaker inhibitor of cytochrome P450 2D6 (CYP2D6) than paroxetine, and thus only slightly reduces plasma concentrations of endoxifen, the potent tamoxifen metabolite (Pritchard 2010) (This is discussed in further detail under 4.2, Safety of Antidepressants in Patients with a History of Breast Cancer, and 4.2.3, Interactions with Tamoxifen).