On a concentration-time graph, onset of action is best defined as the time to reach MEC.

On a concentration-time graph, onset of action is best defined as the time to reach MEC.

• A. Time to peak concentration
• B. Time to reach MEC
• C. Area under the curve
• D. Duration above MEC

Answer: (B)

Onset of action means when the drug starts to produce a detectable effect. On a concentration–time graph, this happens at the moment the drug concentration first reaches the minimum effective concentration (MEC), the threshold needed to elicit a response in the target tissue. Before crossing MEC, there isn’t enough drug to trigger the effect; after crossing MEC, the effect begins and typically grows with higher concentrations until it levels off. Time to peak concentration reflects how long it takes to reach the maximum concentration (Tmax), which is not when the effect starts. Area under the curve represents total drug exposure over time, not the starting point of action. Duration above MEC describes how long the concentration stays above the threshold, relating to how long the effect lasts, not when it begins. Therefore, the best definition of onset is the time to reach MEC.

Onset of action means when the drug starts to produce a detectable effect. On a concentration–time graph, this happens at the moment the drug concentration first reaches the minimum effective concentration (MEC), the threshold needed to elicit a response in the target tissue. Before crossing MEC, there isn’t enough drug to trigger the effect; after crossing MEC, the effect begins and typically grows with higher concentrations until it levels off.

Time to peak concentration reflects how long it takes to reach the maximum concentration (Tmax), which is not when the effect starts. Area under the curve represents total drug exposure over time, not the starting point of action. Duration above MEC describes how long the concentration stays above the threshold, relating to how long the effect lasts, not when it begins. Therefore, the best definition of onset is the time to reach MEC.