Oxidative metabolism by cytochrome p450 enzymes is a primary method of drug metabolism. The purpose of drug metabolism is to make drugs more water soluble so they can be more easily excreted from the body. Drug interactions involving the cytochrome p450 system are common, and generally result from either enzyme inhibition or induction. Enzyme inhibition generally involves competition with another drug for enzyme binding sites, and usually begins with the first dose of the inhibitor. Duration of inhibition corresponds to the half-lives of the respective drugs.
Enzyme induction occurs when one drug stimulates production of more enzymatic metabolism capacity.
The effects of enzyme induction are sometimes more difficult to predict because these are dependent on drug half-lives, the rate of enzyme production, and individual genetic variations. Genetic differences are the reason one patient might be susceptible to interactions when another may not. Cytochrome p450 enzymes are often designated by the letters CYP followed by a set of letters and numbers that distinguish enzyme isoforms. Understanding CYP enzyme interactions might allow prescribers the ability to better anticipate and manage each patient's response to a drug regimen. The summary figures below list commonly used drugs affected by CYP interactions.