What is the distribution-phase half-life, and how is it determined?

• A. It is the half-life during the elimination phase and is determined from the terminal slope
• B. It is the half-life during the absorption phase and is determined from the initial rise
• C. It is the half-life during the distribution phase (alpha t1/2) and is determined from the early plasma concentration decline
• D. It is the half-life of overall drug in body

Answer: (C)

The distribution-phase half-life, often called alpha t1/2, is the time it takes for plasma drug concentration to fall by half during the rapid distribution of the drug from the blood into tissues after a dose (usually after an IV bolus). This phase is described by the distribution rate constant α in a two-compartment model, where the concentration-time profile shows a fast initial decline as the drug moves out of plasma and into tissues, followed by a slower elimination phase governed by β.

You determine it from the early part of the concentration–time curve, before elimination dominates. On a semilog plot, this is the steep initial straight-line segment. The relationship is t1/2,α = 0.693/α. In practice, you estimate α from the slope of that early portion (or from early concentration data using a two-exponential fit) and then compute the half-life from that slope.

This is distinct from the elimination half-life, which comes from the slower, long-term decline and reflects overall clearance from the body, not the rapid distribution into tissues.