Elimination half-life is defined as

Elimination half-life is defined as

• A. The time necessary to eliminate 50% of the drug from the body after rapid IV injection
• B. The time for plasma concentration to fall by 80%
• C. The time to reach Css
• D. The duration of the drug's effect

Answer: (A)

Elimination half-life is the time required for the plasma concentration (or the amount in the body) to drop by 50% during the elimination phase. When a drug is given quickly by IV, distribution happens rapidly, and after that, the decline in drug level follows first‑order kinetics, so the half-life remains constant. This means with each half-life you have half as much drug left; after two half-lives you have a quarter, after three a eighth, and so on. This concept is distinct from an 80% reduction, which would take about four half-lives, not the single half-life. It also isn’t the time to reach steady-state concentration during a continuous infusion—that steady-state timing is defined by multiple half-lives but relates to input and elimination balancing out, not the intrinsic half-life itself. And it isn’t the duration of the drug’s effect, which depends on pharmacodynamics in addition to pharmacokinetics.

Elimination half-life is the time required for the plasma concentration (or the amount in the body) to drop by 50% during the elimination phase. When a drug is given quickly by IV, distribution happens rapidly, and after that, the decline in drug level follows first‑order kinetics, so the half-life remains constant. This means with each half-life you have half as much drug left; after two half-lives you have a quarter, after three a eighth, and so on.

This concept is distinct from an 80% reduction, which would take about four half-lives, not the single half-life. It also isn’t the time to reach steady-state concentration during a continuous infusion—that steady-state timing is defined by multiple half-lives but relates to input and elimination balancing out, not the intrinsic half-life itself. And it isn’t the duration of the drug’s effect, which depends on pharmacodynamics in addition to pharmacokinetics.