Which equation gives the oral dose needed to achieve a target Css when clearance, dosing interval, and bioavailability are known?

• A. Dose_po = (Css × Cl × τ) / F
• B. Dose_po = Css × Cl × τ × F
• C. Dose_po = (Css × Cl) / (τ × F)
• D. Dose_po = F × Css × Vd

Answer: (A)

When aiming for a target average steady-state concentration with intermittent oral dosing, the balance between how much drug enters the body and how quickly it is cleared is the key idea. At steady state, the average rate of input must match the average rate of elimination. The average input rate for oral dosing is the amount absorbed per dose divided by the dosing interval, which is (F × Dose)/τ, since only a fraction F of the administered dose reaches systemic circulation and doses are given every τ hours. The average elimination rate at steady state is Css × Cl. Setting these equal gives Dose = Css × Cl × τ / F. This shows why the dose increases with clearance and with the dosing interval, and why it decreases with higher bioavailability. The given expression matches this relationship, yielding the required oral dose to achieve the target Css.