Most metabolic biotransformations occur at some point between absorption of the drug into the general circulation and its renal elimination. A few transformations occur in the intestinal lumen (bacterial activity) or intestinal wall (villi). In general, all of these reactions can be assigned to one of 2 major categories:

Phase I reactions usually convert the parent drug to a more polar metabolite by introducing or unmasking a functional group (-OH, -NH2, -SH). Often these metabolites are inactive, although in some instances activity is only modified. If Phase I metabolites are sufficiently polar, they may be readily excreted.

Many Phase I products are not eliminated rapidly and undergo a subsequent reaction in which an endogenous substrate such as glucuronic acid, sulfuric acid, acetic acid, or an amino acid combines with the newly established functional group to form a highly polar conjugate. Such conjugation or synthetic reactions are the hallmarks of Phase II metabolism.