What characteristic distinguishes subcutaneous drug administration from intravenous?

Subcutaneous drug administration is distinguished by slower and more sustained absorption compared to intravenous administration. This is primarily due to the fact that when a drug is injected subcutaneously, it enters the tissue layer beneath the skin and must diffuse through the interstitial fluid and then through capillary walls to enter the bloodstream. This diffusion process takes time, which results in a gradual increase in drug concentration in the plasma.

In contrast, intravenous administration delivers the drug directly into the bloodstream, leading to immediate availability and rapid peak concentrations. The slower absorption from subcutaneous administration can be beneficial for certain medications, providing prolonged effects and reducing the likelihood of peak-related side effects.

Other options primarily relate to characteristics of drug administration methods: larger volumes are typically needed for intravenous routes but not necessarily for subcutaneous; and while intravenous administration guarantees 100% bioavailability due to direct entry into circulation, subcutaneous routes do not provide this level of bioavailability due to factors like absorption variability.