Which route of administration has the quickest onset of action?

The intravenous (IV) route of administration has the quickest onset of action because it delivers the drug directly into the bloodstream. This ensures immediate availability of the medication at the site of action without the need for absorption across biological membranes. Since the drug is already in circulation, it can exert its effect almost instantly or within just a few minutes, making IV administration particularly useful in emergency settings where rapid therapeutic effects are paramount.

In contrast, other routes such as intramuscular (IM) and subcutaneous (SC) require the drug to be absorbed into the bloodstream, which takes more time. The oral route is generally the slowest due to the necessity for the drug to be dissolved in the gastrointestinal tract, absorbed through the gut barrier, and then transported through the liver before reaching systemic circulation. Hence, the IV route stands out for its ability to provide rapid onset of drug action.