Drug X has a half-life of 8 hours and is given every 8 hours (one half-life interval). Approximately how many doses are required to reach steady-state plasma concentrations?

• A 2 doses
• B 10 doses
• C 5 doses ✓
• D Immediately after the first dose due to the dosing interval equalling the half-life

Explanation

Steady-state (Css) is reached after approximately 4–5 half-lives regardless of the dosing interval. After 1 half-life, 50% of Css; after 2 half-lives, 75%; after 3 half-lives, 87.5%; after 4 half-lives, 93.75%; after 5 half-lives, ~97%, which is clinically considered steady state. With a half-life of 8 hours and dosing every 8 hours, steady state is reached in approximately 5 doses (corresponding to ~5 half-lives or 40 hours). The loading dose concept exists precisely to achieve Css faster than waiting 5 half-lives.

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

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