ED50 and EC50 refer to the dose or concentration required to produce what percentage of the maximum response?

• A. The maximal response produced by the drug.
• B. The dose/concentration required to produce 50% of the maximum response.
• C. The time to reach half of the maximal response.
• D. The potency at which receptors are noncompetitive.

Answer: (B)

ED50 and EC50 are the dose or concentration that achieves half of the maximum effect. They are measures of potency on the dose–response curve, pointing to the amount needed to reach 50% of Emax (the maximal response). A smaller ED50/EC50 means higher potency because less drug is required to reach that half-max level. This concept is about how much drug is needed to achieve a given fraction of the maximum response, not about the maximal effect itself, the time to reach half-max, or noncompetitive aspects.