A new antibiotic has a clearance of 2.5 L/h and a volume of distribution of 35 L in an average adult. What is its elimination half-life?
- A Approximately 9.7 hours ✓
- B Approximately 4.9 hours
- C Approximately 14 hours
- D Approximately 19.4 hours
Correct answer: A. Approximately 9.7 hours
Explanation
Half-life equals 0.693 times Vd divided by clearance. Substituting, 0.693 multiplied by 35 gives 24.255, and dividing by 2.5 L/h yields approximately 9.7 hours. Choosing 4.9 hours results from omitting the 0.693 factor, while 19.4 hours comes from multiplying rather than dividing by clearance. This relationship explains why a larger Vd or slower clearance lengthens the dosing interval needed for intermittent regimens.
Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th ed.
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