Compounds begin to break down as soon as they enter the body.  The majority of small-molecule drug metabolism is carried out in the liver by redox enzymes, termed cytochrome P450 enzymes.  As metabolism occurs, the initial (parent) compound is converted to new compounds called metabolites.  When metabolites are pharmacologically inert, metabolism deactivates the administered dose of parent drug and this usually reduces the effects on the body.  Metabolites may also be pharmacologically active, sometimes more so than the parent drug (see prodrug).