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Bioavailability
Bioavailability
Overview
- Bioavailability refers to the percentage of drug that reaches the systemic circulation unchanged.
- In oral administration, bioavailability accounts for the losses due to poor drug absorption via the gut and the first past effect (liver clearance).
Formula
- F = f x (1 – ER)
Variables
- F = Systemic Bioavailability
- f = Extent of Absorption
- ER = Extraction Ratio
Intravenous & Oral Administration: Physiologic Diagram
- We can imagine a patient who is having painful muscle spasms in the arm and we must get drug to the arm muscle membrane to reduce contractility.
Intravenous Administration
- We can administer the medication (eg, diazepam) intravenously.
- The venous circulation empties into the heart and then passes into the arterial circulation to reach the target site.
- In this situation, the drug immediately enters the systemic circulation without any barriers or metabolism and so the bioavailability is 100%.
- 100% of the drug reaches the systemic circulation unchanged.
IV Biovailability
- By definition, IV bioavailability is always 100%.
Oral Administration (PO)
- The stomach, small intestine, and pancreas are key to GI absorption.
Key Factors in Oral Biovailbility
- Gut absorption
- First pass effect
- Hepatic portal blood flow
Bioavailability Range
- Oral bioavailability is wide-ranging; from minimal bioavailability (5%) to great (95%).
- For diazepam (Valium), there is ~ 98% PO bioavailability, thus we administer a similar dose orally as we do IV (eg, typically 5-10 mg IV or PO) because the bioavailabilities are roughly the same.
- The main difference is the time of action: IV administration takes 5-10 minutes whereas oral administration takes 1-2 hours.