MEDizzy - We are planning to infuse a drug intravenously at a constant

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General pharmacology

We are planning to infuse a drug intravenously at a constant amount per unit time (rate). It has a first-order elimination rate constant (kel) of 0.35 h−1 . No loading dose will be given. Approximately how long will it take for blood levels to reach steady state after the infusion begins?

ExplanationWith first-order elimination of a drug, we get a straight line if we plot the log of drug concentration in the blood vs. time. The slope of the line is the elimination rate constant (kel). The drug’s half-life—a value we will need to use momentarily—is related to kel as follows: t1/2 × (kel) = 0.693. So, for the drug noted in the question, t1/2 = 0.693/0.35, or approximately 2 h. It takes approximately 4–5 half-lives to reach a steady-state blood concentration. Do the simple math and you will see that the time for this drug to reach a steady state is approximately 8–10 h, and the answer given is therefore the best answer.