(1) A muscle which contracts and causes a movement. Contraction of an agonist is complemented by relaxation of its antagonist (see below).
(2) A drug that acts through receptors on the surface of the cell or within the cell and provokes a biological response. As the body contains natural agonists that combine with cell receptors, any ‘occupation’ of these cell receptors by drug molecules will have a pharmacological eﬀect on the individual. The intensity of that pharmacological eﬀect is believed to be directly proportional to the number of receptors on the cell that combine with the drug molecule. For example, the natural agonist noradrenaline contracts the smooth muscle of blood vessels; the drug agonist phenylnephrine has a similar eﬀect.
Antagonists are drugs which will combine with the receptor site to prevent another agent from producing its greatest eﬀect. If the drug has no eﬃcacy of its own, but simply prevents the agonist from acting at the receptor site, it is called a full antagonist. A partial antagonist is a drug that provokes some activity at the receptor site. An example of an antagonist is prazosin, which acts against the natural agonist noradrenaline at the receptor site of the cells of blood-vessel muscle and prevents the vascular muscle from contracting.