Which statement best defines the duration of action for a drug on a concentration-time graph?

Which statement best defines the duration of action for a drug on a concentration-time graph?

• A. Time above MEC
• B. Time to reach MEC
• C. Time from onset to full elimination
• D. Time to below MEC

Answer: (A)

The duration of action is defined by how long the drug concentration stays above the minimum effective concentration (MEC). On a concentration-time graph, MEC is a threshold line; the drug’s therapeutic effect is expected any time the curve sits above this line. Therefore, the duration of action corresponds to the time interval during which the concentration remains above MEC. This is why the best descriptor is time above MEC: it directly marks how long the drug is expected to exert a therapeutic effect. Onset of action would be the moment the curve first crosses above MEC, while the end of effect occurs when it falls back below MEC.

The duration of action is defined by how long the drug concentration stays above the minimum effective concentration (MEC). On a concentration-time graph, MEC is a threshold line; the drug’s therapeutic effect is expected any time the curve sits above this line. Therefore, the duration of action corresponds to the time interval during which the concentration remains above MEC. This is why the best descriptor is time above MEC: it directly marks how long the drug is expected to exert a therapeutic effect. Onset of action would be the moment the curve first crosses above MEC, while the end of effect occurs when it falls back below MEC.