A drug has a half-life of 6 hours and is given every 6 hours (one half-life interval). After how many doses will steady-state plasma concentration be approximately achieved?

• A After exactly 2 doses
• B After 10 half-lives
• C After 1 half-life, if the loading dose equals twice the maintenance dose
• D After 5 half-lives (approximately 30 hours, i.e., 5 doses) ✓

Explanation

Steady state is reached when the rate of drug input equals the rate of elimination, which occurs after approximately 4–5 half-lives regardless of dosing interval or dose size. At 5 half-lives, approximately 97% of steady state is achieved. Since the half-life is 6 hours and each dose interval is 6 hours, this means ~5 doses are required. A loading dose can immediately achieve target concentrations but does not change the time to true steady state on maintenance dosing.

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

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