ED50/EC50 describe what pharmacodynamic measure?

• A. The maximal response produced by the drug.
• B. The dose/concentration required to produce 50% of the maximum response.
• C. The dose to produce 100% of the max response.
• D. The time to reach half of the maximal response.

Answer: (B)

ED50 and EC50 are about potency: they describe the dose or concentration needed to achieve half of the maximal pharmacologic effect. On a dose–response curve, the response rises with dose until it levels off at the maximal effect (Emax). The point where you reach 50% of that max is the ED50 (dose in vivo) or EC50 (concentration in vitro). A lower value means greater potency because less drug is required to reach half-maximal effect. This is different from Emax, which is the maximal effect itself, and from time-based measures like half-life.