Compared with oral administration, an intramuscular injection would shift the concentration-time curve in which direction?

• A. Right
• B. No shift
• C. Upward
• D. Left

Answer: (D)

Absorption rate controls when the peak concentration is reached on a concentration–time curve. Injecting into muscle (intramuscular) delivers the drug directly into well‑perfused tissue, so it enters the bloodstream more quickly than a drug taken by mouth, which must first dissolve in the GI tract and often undergo slower, more variable absorption. Because absorption is faster with intramuscular administration, the blood concentration rises sooner and the peak occurs earlier, shifting the curve to the left on the time axis. The overall exposure and peak height can vary with bioavailability, but the key directional change is an earlier Tmax reflected as a leftward shift. A rightward shift would mean slower absorption than oral, which isn’t typical for IM, and an upward shift would imply higher concentrations across time regardless of timing, which doesn’t capture the route‑related difference.