A drug with a bioavailability of 40% and a clearance of 5 L/h is administered orally at 100 mg every 8 hours (τ = 8 h) at steady state. What is the average steady-state plasma concentration (Css,avg)?

• A 2.0 mg/L
• B 0.5 mg/L
• C 1.0 mg/L ✓
• D 4.0 mg/L

Explanation

Css,avg = (F × Dose) / (Cl × τ) = (0.40 × 100 mg) / (5 L/h × 8 h) = 40 mg / 40 L = 1.0 mg/L. This formula directly relates bioavailability, dose, clearance, and dosing interval to the average steady-state concentration. A lower clearance or higher bioavailability raises Css,avg; a shorter dosing interval also increases it, allowing rational dose adjustment in clinical practice.

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

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