A drug has a hepatic extraction ratio (E) of 0.9. Its oral bioavailability will be approximately:
- A 10% ✓
- B 90%
- C 50%
- D 45%
Correct answer: A. 10%
Explanation
Oral bioavailability after hepatic first-pass = F = 1 − E = 1 − 0.9 = 0.1 = 10%. High hepatic extraction ratio drugs (E > 0.7, e.g., lidocaine, propranolol, morphine, nitrates, isosorbide dinitrate) undergo extensive first-pass hepatic metabolism, leaving only ~10% (or less) of the oral dose reaching systemic circulation unchanged. These drugs typically require much higher oral doses than IV doses, or must be given by alternate routes (sublingual, transdermal, IV) to achieve adequate plasma levels.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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