To be useful as a medicine, a drug must be capable of being delivered to its site of action in a concentration large enough to initiate a pharmacological response. This concentration will depend on the amount of drug administered, the rate and extent of its absorption and its distribution in the blood stream to other parts of the body. The medicine will continue to act until the concentration of drug drops below its threshold for pharmacological activity either due to its removal (excretion) from the body in an unchanged form or after

its metabolism to a more polar substance. The interrelationship between the absorption, distribution, metabolism and excretion of a drug is referred to as pharmacokinetics and describes how drugs are handled by the body. Such knowledge of a new drug is fundamental to the drug development process, to enable selection of the optimal dose, route and frequency of dosing to produce the desired clinical effect, without producing unwanted side-effects.