Which phase in a two-compartment model is associated with elimination?

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

Answer: (B)

In a two-compartment model, after an IV dose you see two distinct parts of the concentration-time curve: an initial, rapid drop as the drug moves between the central and peripheral compartments, and a later, slower decline as the drug is eliminated from the body. The phase that reflects elimination is the slower, terminal portion—the beta phase. This phase corresponds to first-order elimination from the central compartment, and on a semi-log plot its slope equals the elimination rate constant (beta). The rapid alpha phase is driven by intercompartmental transfer (distribution) rather than clearance. Gamma and delta phases aren’t part of the standard two-compartment framework. So, the elimination phase is the beta phase.