A drug follows first-order elimination kinetics with an elimination half-life of 6 hours. If a single IV bolus dose is administered, approximately what percentage of the original dose remains in the body after 18 hours?

• A 50 percent
• B 25 percent
• C 12.5 percent ✓
• D 6.25 percent

Explanation

In first-order kinetics a constant fraction of the drug is eliminated per unit time. Eighteen hours equals three half-lives (6 x 3). After one half-life 50 percent remains, after two half-lives 25 percent, and after three half-lives 12.5 percent remains. The distractor B tempts students who count only two half-lives by dividing 18 by 6 incorrectly or stopping one step early.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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