An agonist is a substance that binds to a receptor and triggers a response in the cell. An agonist is the opposite of an antagonist in the sense that while an antagonist also binds to the receptor, it fails to activate the receptor and actually blocks it from activation by agonists. A partial agonist activates a receptor but does not cause as much of a physiological change as does a full agonist. The receptors of the human body work by being stimulated or inhibited by natural (such as hormones and neurotransmitters) or synthetic (such as drugs) agonists and antagonists.
For example, in treating Parkinson disease, the long-used drug levodopa can cause uncontrollable, jerky body movements called dyskinesias that can inhibit a person's ability to function. Dopamine agonists mimic the effects of dopamine in the brain by stimulating dopamine receptors with a lower risk of the uncontrollable and irreversible dyskinesias often associated with levodopa therapy. The word "agonist" comes from the Late Latin agnista, contender, from the Greek agnists, contestant, from agn, contest. An agonist is a chemical contestant or contender.