This minireview Summarizes information concerning the differences and similarities of the human flavin-containing- (FMO, E.C. 1.14.13.8) and the cytochrome P450-monooxygenases (CYP, E.C. 1.14.14.1). Human FMO oxygenates soft nucleophiles. CYP mainly catalyzes C-H abstraction but also oxidizes nitrogen- and sulfur-containing compounds. Both FMO and CYP generally convert lipophilic compounds into more hydrophilic metabolites. The mechanism by which each monooxygenase operates is quite distinct. Sometimes, CYP or FMO bioactivate chemicals to reactive metabolites but to date, drug toxicity thus far observed in the clinic is mainly, the result of CYP-dependent oxidation. Both FMO and CYP possess genetic variability that may contribute to inter-individual variability observed for drug metabolism. In contrast to CYP, FMO is not induced Or readily inhibited and potential adverse drug-drug interactions are minimized for drugs prominently metabolized by FMO. These properties may provide advantages in drug design, and by incorporating FMO detoxication pathways into drug candidates, more drug-like materials may emerge. (c) 2005 Elsevier Inc. All rights reserved.