ED50 or EC50 is best defined as the dose or concentration required to produce 50% of the maximum response.

• A. The dose or concentration required to produce 50% of the maximum response or effect of the drug.
• B. The dose that causes death in 50% of subjects.
• C. The dose at which 50% of patients reach a therapeutic threshold.
• D. The concentration at which 50% of receptors are occupied.

Answer: (A)

The key idea is the dose–response relationship: the point on the curve where the response is half of its maximum defines potency. ED50 or EC50 is the dose or concentration that produces 50% of the maximal effect (Emax). This is a standard way to quantify how much drug is needed to achieve half of what the drug can potentially do, and it reflects potency—the smaller the ED50/EC50, the more potent the drug.

This definition is not about lethal dose (LD50), nor about a therapeutic threshold reached by 50% of patients specifically, and it’s not simply about receptor occupancy. While occupancy can relate to the effect, ED50/EC50 figures describe the functional response or effect level, not just binding.