All newly developed drugs coming onto the market have been tested with regard to their absorption, distribution, metabolism and excretion/clearance (ADME testing). Thus, early evaluation of pharmacokinetics has become pivotal for new drug discovery and lead optimization and considerable effort is expended in new drug research to avoid the development of numerous compounds showing promising pharmacological activity but will eventually fail for reasons related to bioavailability, toxicity, or drug-drug interactions (e.g. activation/inhibition of CYP proteins).

In addition to affecting its own metabolism and clearance rate, drugs can affect the rate of clearance of other, simultaneously administered drugs, causing severe or even fatal drug-drug interactions (DDI). Besides being degraded and cleared, some drugs can also be activated - primarily by liver CYP450 enzymes - to form potent metabolites either with the desired efficacy or with unwanted, adverse side effects.