Question

A narcotics addict is brought to the emergency room in a deep coma. His friends state that he took a large dose of heroin 6 hours earlier. An immediate blood analysis shows a heroin blood level of 0.25 mg/Liter. Assuming that the pharmacokinetics of heroin in this patient are VD = 200 Liters and half-life = 3 hours, how much heroin did the patient inject 6 hours earlier?

Answer 1

Initial concentration = Dose /Vd

VD= 200

Half-life = 3 hours (Serum half-life – time required for serum concentrations to decrease by one half)

Blood level at 6 hours= 0.25mg/dl

Vd= Dose/ plasma concentration at given time

200L=Dose/ 0.25 mg/L at 6 hours

Dose at 6 hours =200 L x 0.25 mg/L = 50 milligrams at 6 hours

Two half-lives elapsed till 6 hours, so the percentage of the drug remains is 25% or 1/4^th of the initial concentration i.e. 50 milligrams

The initial concentration is 50 milligrams x 4 = 200 milligrams

The drug taken was 200 milligrams