Which statement is correct regarding onset of drug action?

Which statement is correct regarding onset of drug action?

• A. Onset reflects time to minimum effective concentration (MEC), not full therapeutic concentration.
• B. Onset equals time to full therapeutic concentration
• C. Onset is always shorter than MEC
• D. Onset has no relation to concentration

Answer: (A)

Onset of action is the time from administration until the drug begins to produce a detectable effect. This occurs when the drug concentration at the site of action reaches the minimum amount needed to generate a therapeutic response—the minimum effective concentration (MEC). Once the concentration reaches MEC, pharmacodynamic processes are triggered and the effect starts; further increases in concentration raise the effect toward its full level, which takes additional time. So the onset reflects time to MEC, not the time to reach the full therapeutic concentration. The other statements misstate the relationship by implying onset equals full therapeutic concentration, or that onset is unrelated to concentration, or that onset is always shorter than MEC.

Onset of action is the time from administration until the drug begins to produce a detectable effect. This occurs when the drug concentration at the site of action reaches the minimum amount needed to generate a therapeutic response—the minimum effective concentration (MEC). Once the concentration reaches MEC, pharmacodynamic processes are triggered and the effect starts; further increases in concentration raise the effect toward its full level, which takes additional time. So the onset reflects time to MEC, not the time to reach the full therapeutic concentration. The other statements misstate the relationship by implying onset equals full therapeutic concentration, or that onset is unrelated to concentration, or that onset is always shorter than MEC.