EC50 is commonly described as which of the following?

• A. The dose required to produce 50% of the maximum effect
• B. The time to reach 50% of the maximum effect
• C. The concentration of drug that produces 50% of its maximal effect
• D. The maximum effect achievable by the drug

Answer: (C)

EC50 is the concentration of a drug that produces 50% of its maximal effect on a dose–response curve. This makes it a measure of potency: drugs that need a smaller concentration to reach half of the maximal response are more potent. It focuses on how much of the drug is required to achieve half of Emax, not on time to effect or on the maximum effect itself. The maximum effect is described by Emax, while a dose-based metric would be ED50, which refers to the dose needed to achieve 50% of the maximum effect.