Cytochrome P450 (CYP) enzymes are common sites of drug interactions in human beings. Drugs may act as inhibitors or inducers of CYPs, leading to altered clearance of a second drug. Clinically relevant drug interactions involving various CYP isoforms in people, including CYP1A2, CYP2C9, CYP2D6, and CYP3A4, have been well documented. Analogous interactions are beginning to be characterized in dogs, for which canine CYPs share many of the same substrate ranges as in human beings.