In pharmacology, what does EC50 stand for?

• A. The concentration of drug that produces 50% of its maximal effect
• B. The amount of drug in the bloodstream at steady state
• C. The time to reach 50% of maximal effect
• D. The dose producing 50% of maximum effect

Answer: (A)

EC50 is the concentration of a drug that produces 50% of its maximal effect. It tells you about potency, not the maximum effect itself. On a dose–response curve, the EC50 point is where the response reaches half of its greatest possible level. A lower EC50 means the drug is more potent because you need less concentration to reach half of the maximal response. This value depends on the system and conditions (receptor availability, assay setup, etc.), so it’s about concentration driving effect in a given context, not about how much drug is circulating at steady state or how long it takes to elicit a response, and not about the dose that yields half of the maximum effect in vivo (that latter concept is ED50).