Onset of action refers to which of the following?

• A. The time from administration to first measurable effect
• B. The time to reach peak effect
• C. The time from administration to the beginning of the therapeutic effect
• D. The duration of the drug's presence in the body

Answer: (C)

The key idea is the time from when a drug is given until it starts producing its therapeutic effects. Onset of action tracks when the drug first begins to work at a clinically meaningful level, which depends on how quickly the drug is absorbed and reaches its site of action and how fast it triggers the therapeutic response. That’s why this option—time from administration to the beginning of the therapeutic effect—best fits the concept.

The other ideas describe different phases: time to first measurable effect might occur before the therapeutic effect and isn’t guaranteed to be clinically meaningful; time to reach peak effect is about when the maximum effect occurs, not when it starts; and duration refers to how long the drug remains active in the body, not when its action begins.