Which factor most influences the onset of action after extravascular administration?

• A. Ambient Temperature
• B. Route of Administration
• C. Molecular Weight of Drug
• D. Color of Solution

Answer: (B)

The main point is that onset after extravascular dosing is driven primarily by how quickly the drug is absorbed into the bloodstream. The route of administration sets the major barriers and pathways to absorption. For example, oral dosing must first dissolve in the GI fluids, then cross the intestinal lining, and may undergo first-pass metabolism in the liver, which can delay or reduce the amount reaching systemic circulation. In contrast, injections into tissue (such as subcutaneous or intramuscular) bypass the GI tract and first-pass effects; their onset depends more on local blood flow, the drug’s solubility, and its formulation, which can allow faster absorption in well-perfused tissues. Therefore, the route of administration most strongly influences how quickly the drug starts acting after extravascular dosing.

Ambient temperature can slightly affect dissolution but does not determine onset as much as the chosen route. Molecular weight and drug formulation influence absorption, but the route remains the primary factor. Color of the solution has no bearing on absorption or onset.