A drug with a half-life of 6 hours is given as a single intravenous bolus. What percentage of the administered dose remains in the body after 18 hours?

• A 25 percent
• B 6.25 percent
• C 12.5 percent ✓
• D 37.5 percent

Explanation

Eighteen hours equals exactly three half-lives of this drug. After each half-life, 50 percent of the remaining drug is eliminated: 100 to 50 to 25 to 12.5 percent. The common error is confusing the number of half-lives elapsed with the percentage remaining, choosing 37.5 percent by subtracting three times 12.5 from 100 instead of halving sequentially.

Reference: Katzung Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.