A drug is administered intravenously with a half-life of 8 hours. Approximately how long will it take to reach steady-state plasma concentration with a fixed-interval dosing regimen?

• A 40 hours ✓
• B 8 hours
• C 16 hours
• D 80 hours

Explanation

Steady state is reached after approximately 4-5 half-lives regardless of dose or dosing interval. With a half-life of 8 hours, 5 x 8 = 40 hours. At this point, the rate of drug input equals the rate of elimination. Neither doubling the dose nor changing the dosing interval alters the time to steady state — only the plasma level at steady state changes.

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

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