In a two-compartment model, which phase describes distribution into peripheral tissues?

• A. Alpha phase
• B. Beta phase
• C. Gamma phase
• D. Delta phase

Answer: (A)

In the two-compartment model, the rapid movement of drug from the central (blood) compartment into peripheral tissues happens during the distribution phase, known as the alpha phase. Right after an IV dose, plasma concentrations fall quickly because the drug is distributing into tissues, a process governed by the intercompartmental rate constants. This phase is typically much faster than elimination, which becomes apparent later as the beta phase, where the drug is cleared from the body at a slower rate. Phases named gamma or delta appear only in more complex models with additional compartments, not in the standard two-compartment setup. So the phase describing distribution into peripheral tissues is the alpha phase.