At 12 h after IV administration of a bolus dose, the plasma level of a drug is 3 mg/L. If the Vd=10 L and the elimination half-life = 6 h, what was the dose administered?

At 12 h after IV injection (which corresponds to two half-lives of the drug), the plasma level is 3mg/L. Extrapolating back to zero time, by "doubling" plasma level for each half-life, results in an initial plasma level at zero time (C0)=3x2x2mg/L=12mg/L.Doseinjected=Vd xC0=10Lx12mg/L=120mg