An agonist's a term used to describe a type of ligand or drug that binds and alters the activity of a receptor. The ability to alter the activity of a receptor, also known as the agonist's efficacy,'s a property that distinguishes it from antagonists, a type of receptor ligand which also binds a receptor but which doesn't alter the activity of the receptor. The efficacy of an agonist may be positive, causing an increase in the receptor's activity or negative causing a decrease in the receptor's… (More on Agonist)