A drug has a half-life of 6 hours. If the plasma concentration is currently 100 mg/L, what will the concentration be after 18 hours, assuming first-order kinetics and no further doses?
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Correct answer: 12.5 mg/L
Eighteen hours represents three half-lives ($18 / 6 = 3$). After one half-life it is 50, after two it is 25, and after three half-lives, the concentration is 12.5 mg/L.
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