In pharmacology, EC50 is best described as which of the following?

• A. The time to reach half of the maximum concentration
• B. The dose that produces 50% of the maximum effect
• C. The concentration that produces 50% of the maximal effect
• D. The rate of drug clearance at steady state

Answer: (C)

EC50 is the concentration of a drug that produces 50% of its maximal pharmacologic effect. This value reflects potency: a smaller EC50 means the drug is more potent because less of it is needed to reach half of the maximum response. You derive EC50 from a concentration–response curve, where the response increases with concentration and levels off at the maximum effect (Emax). At the EC50 point, the observed effect equals half of Emax.

It’s different from a time-related measure like half-life or a dose-based measure like ED50, and it isn’t about how fast the drug is cleared. For inhibitory processes, a similar term is IC50, the concentration that inhibits 50% of the response. In practice, comparing EC50 values helps you understand which drug achieves a given level of effect at a lower concentration.