A drug with first-order kinetics has a half-life of 6 hours. Approximately what fraction of the initial dose remains in the body after 24 hours?
- A 1/8 (12.5%)
- B 1/4 (25%)
- C 1/16 (6.25%) ✓
- D 1/32 (3.125%)
Correct answer: C. 1/16 (6.25%)
Explanation
In first-order kinetics, drug amount halves every half-life. After 24 hours with a 6-hour half-life, 4 half-lives have elapsed: (1/2)^4 = 1/16 = 6.25% of the initial dose remains. After 1 half-life: 50%; after 2: 25%; after 3: 12.5%; after 4: 6.25%. By convention, ≈97% of the drug is eliminated after approximately 5 half-lives.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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