Why may there be a time delay even if the drug is fully absorbed?

• A. Must reach equilibrium at the site of action
• B. It is rapidly metabolized in the liver
• C. It binds to plasma proteins delaying effect
• D. It is excreted before reaching the site

Answer: (A)

Onset of effect hinges on distribution to the site of action, not just absorption. After the drug is fully absorbed, it must move from the bloodstream into the tissues where it works and reach concentrations high enough to produce a response. This distribution, often described by a central–peripheral (two-compartment) model, takes time as the drug equilibrates between blood and tissues. The delay you observe—the time before the pharmacologic effect appears—occurs because the tissue concentration must reach the effective level at the site of action. Factors like tissue perfusion and the drug’s ability to partition into the target tissues govern this lag. While rapid metabolism, plasma protein binding, or excretion can influence overall exposure, the primary reason for a time delay despite full absorption is the time required to reach equilibrium at the site of action.