An agonist is a drug that binds and activates the receptor to elicit a cellular response similar to the endogenous ligand. In a drug-receptor interaction model, a receptor exists in an equilibrium of two conformations: inactive or Ri and active or Ra. The Ri form produces no effect even when bound to a drug, while Ra shows constitutive activity; that is, it can produce a small effect even without drugs. A drug's relative affinity for the Ri or Ra forms determines its efficacy in causing an effect. In the order of decreasing efficacy, agonists can be full, partial, or inverse. Full agonists have a strong affinity for the Ra form and produce a maximal response by occupying fewer available receptors. Partial agonists have an intermediate affinity for Ra and Ri forms. Even with full occupancy of all receptors, they only produce a submaximal response. Lastly, inverse agonists have a stronger affinity for the Ri conformation and stabilize the resting state of the receptors. So, their effect is opposite to that of an agonist.