A patient comes 6 hours after consuming morphine and presents with pin point pupils and respiratory depression. T 1/2 of morphine is 3 hours and volume of distribution (Vd) is 200 L. Current plasma concentration of morphine is 0.5 mg/L. Calculate the initial morphine dose consumed by the person.

* Volume of distribution means how much amount of plasma is required to accommodate the whole drug, if the plasma concentration remains constant. Current plasma concentration is 0.5 mg/L means* 1 litre contains 0.5 mg of morphine So, Vd of 200 L means* 200 litre contain 0.5 x 200=100 mg So now total amount of morphine present in the body is 100 mg. This is the amount present in body after 2 half lives (half life is 3 hours and patient presented after 6 hours)* T1/2 = 3 hrsAfter 1 t1/2 drug remaining in body is 50 % of initial. After 2 t1/2 drug remaining in body is 25 % of initial If initial amount of morphine in body is x mg so, 100 = 25% of x S0, x = 400 mg So the drug present in body initially is 400 mg