Question

You want to achieve a particular plasma concentration of a drug of approximately 81 mg/L in a 70 kg adult male. You know that the total volume of distribution for this drug is ~9L. What dose of drug should you give? (Choose the closest approximate value.)

5670 mg

729 mg correct

10 mg

9 mg

In order to achieve the same plasma concentration of the above drug in an adult male who is obese (weighing 125 kg) but otherwise healthy, what dose of drug should be administered?

1. a higher dose

2. a lower dose

and why ?

Answer 1

1)

Volume of distribution for the Drug 9L

Plasma Concentration is 81 mg/L

The formula for Total Mass of drug needed

Mass of drug= Volume of distribution * Plasma Concentration = (9 L) * (81 mg/L)= 729 mg

2)

For an obese person, Volume of Distribution is higher because of substantially more extra fat present in the body. Thus, as can be seen from the equation used above, Mass of Drug needed is proportional to the Volume of distribution. Thus, for an obese person, a higher dose of drug will be needed.