A drug is administered at regular intervals without a loading dose. How many half-lives are required to reach approximately 97% of steady-state plasma concentration?

• A 2 half-lives
• B 3 half-lives
• C 5 half-lives ✓
• D 8 half-lives

Explanation

After each half-life, the plasma concentration progresses toward steady state by the remaining gap fraction. After 1 half-life: 50% of steady state. After 2: 75%. After 3: 87.5%. After 4: 93.75%. After 5: approximately 97%. This is independent of dose, dosing interval, and route. A loading dose bypasses this delay when rapid therapeutic levels are needed, as with digoxin or phenytoin.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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