Opioids · Pharmacology

What is the primary reason for the rapid termination of action of a single intravenous bolus of Fentanyl?

  1. Rapid hepatic metabolism by CYP3A4 to inactive metabolites
  2. Redistribution from the brain to skeletal muscle and fat
  3. Excretion of the unchanged drug in the urine
  4. Spontaneous hydrolysis by esterases in the plasma
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Correct answer: Redistribution from the brain to skeletal muscle and fat

Fentanyl is highly lipophilic and enters the brain quickly. However, its effect wears off rapidly after a single dose because it redistributes down its concentration gradient into less perfused peripheral tissues, even before significant metabolism occurs.

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