Which statement best describes EC50 in pharmacology?

Which statement best describes EC50 in pharmacology?

• A. The concentration that produces 50% of the maximal effect.
• B. The dose at which 50% of subjects die.
• C. The dose at which 50% of receptor sites are occupied.
• D. The time to reach half-maximal effect.

Answer: (A)

EC50 is the concentration that produces 50% of the maximal pharmacologic effect. On a dose–response curve, it’s the x-value where the response reaches half of Emax, so it directly reflects potency: a drug with a lower EC50 achieves half-maximal response at a lower concentration. This is a pharmacodynamic measure derived from how a drug elicits effect, not a population-wide outcome or a safety/toxicity threshold. It differs from the dose needed to affect 50% of individuals (ED50) and from a time-based half-maximum (t50). It’s also distinct from receptor occupancy, which describes how many receptors are bound by the drug; occupancy relates to binding affinity (Kd) and can differ from the functional response depending on signaling and receptor mechanisms.

EC50 is the concentration that produces 50% of the maximal pharmacologic effect. On a dose–response curve, it’s the x-value where the response reaches half of Emax, so it directly reflects potency: a drug with a lower EC50 achieves half-maximal response at a lower concentration. This is a pharmacodynamic measure derived from how a drug elicits effect, not a population-wide outcome or a safety/toxicity threshold. It differs from the dose needed to affect 50% of individuals (ED50) and from a time-based half-maximum (t50). It’s also distinct from receptor occupancy, which describes how many receptors are bound by the drug; occupancy relates to binding affinity (Kd) and can differ from the functional response depending on signaling and receptor mechanisms.