A drug activates its receptor by inducing conformational changes or recruiting effector molecules to generate a cellular response. However, not all drugs activate their target or receptors upon binding. Drug binding depends on the drug's affinity for the receptor. Even at low concentrations, high-affinity drugs compete with endogenous ligands for maximum receptor occupancy. A drug's efficacy determines its ability to elicit a cellular response. Drugs are categorized as agonists, partial agonists, or antagonists based on their affinity and efficacy. An agonist activates the receptor to elicit a maximal response. Agonists can bind to an endogenous ligand binding site or a separate allosteric site to increase a receptor's activity. Even without total occupancy of receptors, the efficacy of an agonist remains nearly 100%. A partial agonist has intermediate efficacy and can only produce a submaximal response even with 100% receptor occupancy. Finally, an antagonist competes with the endogenous ligand to bind and inactivate the receptor. So, the antagonist has negligible efficacy and does not produce a response.