Following the formation of a drug-receptor complex, the ability of a drug to initiate a response is predicated on the drug’s intrinsic activity. A drug, which interacts with the active binding site of a receptor and has a direct stimulatory effect on that receptor, is called an agonist. A strong agonist produces a significant physiological/pharmacological response when only a relatively small number of receptors are occupied, i.e. the drug has high intrinsic activity.
A weak agonist must interact with the agonist-binding domain of the drug on many more receptors to produce the same effect as a strong agonist, i.e., the drug has lower intrinsic activity. A partial agonist will never produce the same effect as a strong or a weak agonist even when all active binding-sites are occupied by the drug. While a partial agonist has affinity for a receptor, it has low intrinsic activity.