General pharmacology: pharmacokinetics — absorption, distribution, metabolism, excretion
Pharmacology · MBBS
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Study notes
A patient takes propranolol 40 mg orally but needs 160 mg for effect. This is first-pass metabolism — oral propranolol is absorbed, then the liver extracts most of it before it reaches systemic circulation (low bioavailability ~25%). IV propranolol needs far less. Liver disease raises bioavailability dangerously (less extraction); the same dose then overdoses. First-pass explains why some drugs are IV/oral at wildly different doses — the liver taxes oral drugs at the gate.
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