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How do affinity and efficacy affect a drugās potency?
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In pharmacology, the potency of a drug is determined by the combination of its affinity and efficacy. These two factors govern how a drug interacts with receptors to initiate physiological changes:1
- Affinity: This describes the tendency of a drug molecule to bind to a receptor, forming a drug-receptor complex.1
- Efficacy: Also known as intrinsic activity, this refers to the ability of the drug-receptor complex to trigger a physiological response once binding has occurred.1
Together, these properties dictate the drug's activity. For instance, an agonist possesses both sufficient affinity to bind and sufficient efficacy to elicit a cellular response. Conversely, an antagonist has the affinity to bind to a receptor but lacks the efficacy to produce a response, effectively blocking the action of agonists.1
While affinity and efficacy determine potency, it is important to distinguish this from a drug's overall therapeutic utility, which is also influenced by its therapeutic index and margin of safety.2