A drug can be given three possible operational definitions:

• a chemical substance that affects the processes of the body or mind;

• any chemical compound used on or administered to humans or animals as an aid in the diagnosis, treatment or prevention of disease, or other abnormal condition, for the relief of pain or suffering, or to control or improve any physiologic or pathologic state;

• a substance used recreationally for its effects on the central nervous system.

Since the advent of the essential drug concept, essential drugs have been defined by WHO as those that are indispensable and necessary to satisfy the health care needs of the majority of the population. They should therefore be available and accessible at all times, in the appropriate dosage forms and at prices affordable to all. This definition does not in any way diminish the usefulness of any drug that may not be on any given essential drug list. It is a conscious attempt to utilize available resources on the most important drugs of benefit to the majority of the population while recognizing the competition in the public sector.