What term describes the time taken for a medication to start working?

• A. Duration
• B. Onset of action
• C. Peak effect
• D. Bioavailability

Answer: (B)

The time to start a medication’s effects is called the onset of action. This term specifically describes how long it takes from the moment a drug is given until its therapeutic effects begin to appear. It’s the key measure of how quickly relief or a response can be expected after administration.

Onset is different from how long the effect lasts (duration) and from the time to reach the maximum effect (peak effect). It’s also distinct from bioavailability, which is about how much of the dose actually reaches the systemic circulation. In prehospital care, onset helps you anticipate when a patient will begin to improve after a given route of administration—for example, IV drugs often have a faster onset than oral drugs because they enter the bloodstream directly, bypassing delays related to absorption. Even after the drug enters circulation, there can be a short delay before effects are felt as the drug distributes to tissues and receptors.