Which statement best describes the half-life of a drug?

• A. Time required for drug concentration to be reduced by 90%
• B. Time required for drug concentration to be reduced by 50%
• C. Time required for drug concentration to be reduced by 10%
• D. Time required for drug concentration to be reduced by 100%

Answer: (B)

Half-life is the time required for the drug’s plasma concentration to decrease by 50%. This reflects first-order elimination, where a constant fraction of the drug is cleared per unit time, so the amount removed is proportional to what remains. Because of this, after one half-life you have 50% left, after two half-lives you have 25%, after three you have 12.5%, and so on. This concept underpins dosing intervals and how long it takes to reach steady-state or to clear a drug from the body.

The other descriptions describe different reductions, not the defining half-life. A 90% drop would leave 10% of the original concentration and occurs after about 3.3 half-lives. A 10% drop leaves 90% remaining and is not tied to a fixed time like a half-life. A 100% drop would imply complete elimination, which isn’t achievable in finite time with typical first-order kinetics.