Bupropion inhibits cytochrome P450 2D6,5 which is the enzyme that converts venlafaxine into an active metabolite, desvenlafaxine. When CYP2D6 is inhibited, venlafaxine levels rise at the expense of desvenlafaxine. Because the renal excretion of venlafaxine is small, the consequence is increased dose-dependent venlafaxine-related adverse effects.