With the same amount of drug given IM and SubQ, which route has faster onset?

• A. IM will be absorbed faster because it is more vascularized.
• B. SubQ will be faster.
• C. Onset is the same.
• D. Oral will be faster.

Answer: (A)

Absorption rate and onset of action depend on how quickly the drug reaches the systemic circulation, which is mainly driven by blood flow at the administration site. Muscle tissue is more vascularized than subcutaneous fat, with greater capillary density and blood perfusion. Depositing the drug in muscle allows faster entry into the bloodstream, so the time to reach therapeutic levels is shorter and the onset is quicker. Subcutaneous tissue, having less perfusion and a slower diffusion path through fat, releases the drug more slowly, delaying onset. Oral administration adds extra steps—dissolution in the GI tract, absorption across the intestinal lining, and first-pass metabolism—further slowing onset compared with intramuscular delivery. So, with the same dose, intramuscular administration typically produces a faster onset than subcutaneous administration.