Drug disposition refers to all processes involved in the absorption, distribution metabolism and excretion of drugs in a living organism. Once a particular medicine is chosen, the principles of clinical pharmacokinetics are required to ensure the appropriate formulation of drug is chosen for an appropriate route of administration. On the basis of the patient’s drug handling parameters, which require an understanding of absorption, distribution, metabolism and excretion, the dosage regimen for the medicine in a particular patient can be developed. . Metabolism is generally accomplished by phase I enzymes that generate more polar metabolites; the liver is the site of most drug metabolism, although some can occur in extrahepatic loci, such as the gut, kidney, or plasma.