Enhanced CYP2D6 activity has been related to malignancies of the bladder, liver, pharynx, and stomach and, especially, to cigarette-smoking-induced lung cancer. The data suggests that enhanced CYP2D6-mediated metabolism of one or more dietary and other environmental agents, to form a reactive intermediate, plays a role in cancer initiation and/or promotion in various tissues. CYP2D6 polymorphism, which is responsible for the variation in metabolism of debrisoquine 4-hydroxylase, is important in the metabolism of more than 30 drugs and environmental chemicals, including as much as 20% of all commonly prescribed drugs.