How is AUC calculated for an oral dose considering bioavailability?

• A. AUC = Dose / Cl
• B. AUC = (F × Dose) / Cl
• C. AUC = Cl / (F × Dose)
• D. AUC = Dose × F × Cl

Answer: (B)

AUC represents systemic exposure and equals the amount of drug that reaches the circulation divided by the clearance. For an oral dose, only a fraction F of the administered dose actually reaches systemic circulation, due to incomplete absorption and first-pass effects. The amount contributing to systemic exposure is therefore F × Dose. The body clears the drug at a rate determined by clearance, so the AUC becomes (F × Dose) / Cl. If you compare routes, when F = 1 (as with IV dosing), this reduces to Dose/Cl. Conceptually, AUC increases with larger dose, and decreases with higher clearance; it also scales with bioavailability F.

Other formulations would give incorrect results: omitting F overestimates systemic exposure; placing F with clearance or swapping their positions changes the units and the relationship, and multiplying by Cl instead of dividing would not reflect how clearance governs how long the drug stays in the body.