Elimination half time is defined as

Elimination half time is defined as

• A. The time necessary for the plasma concentration to fall by 90% during the distribution phase
• B. The time necessary for the plasma concentration to fall by 50% during the elimination phase
• C. The time required to reach steady-state
• D. The duration of the drug's therapeutic effect

Answer: (B)

Elimination half-time is the time required for the plasma concentration to decline by 50% during the elimination phase. In first-order kinetics, the drug is eliminated at a rate proportional to how much is present, so the concentration falls exponentially and halves with each successive half-life. For a given drug and patient, this half-life depends on clearance and volume of distribution and is given by roughly 0.693 × Vd / CL. The distribution phase happens quickly as the drug moves into tissues and is not described by the elimination half-life. Reaching steady-state takes about four to five half-lives, which is a separate concept from the half-life itself. The duration of the therapeutic effect can extend beyond the half-life depending on factors like active metabolites or tissue sensitivity, so it isn’t defined by elimination half-life alone.

Elimination half-time is the time required for the plasma concentration to decline by 50% during the elimination phase. In first-order kinetics, the drug is eliminated at a rate proportional to how much is present, so the concentration falls exponentially and halves with each successive half-life. For a given drug and patient, this half-life depends on clearance and volume of distribution and is given by roughly 0.693 × Vd / CL. The distribution phase happens quickly as the drug moves into tissues and is not described by the elimination half-life. Reaching steady-state takes about four to five half-lives, which is a separate concept from the half-life itself. The duration of the therapeutic effect can extend beyond the half-life depending on factors like active metabolites or tissue sensitivity, so it isn’t defined by elimination half-life alone.