In a drug with first-order kinetics, if the initial plasma concentration is 200 mg/L and the half-life is 6 hours, what is the plasma concentration after 18 hours?

• A 50 mg/L
• B 25 mg/L ✓
• C 12.5 mg/L
• D 100 mg/L

Explanation

In first-order kinetics, concentration halves every half-life. After 18 hours with t1/2 = 6 hours, 3 half-lives have elapsed (18/6 = 3). Calculation: 200 → (1) → 100 → (2) → 50 → (3) → 25 mg/L. Each half-life reduces the concentration by 50%. After 3 half-lives, 12.5% of the original dose remains: 200 × (0.5)^3 = 200 × 0.125 = 25 mg/L.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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