If the drug follows first-order elimination kinetics, how much of the drug will remain 6 hours after its administration?

A patient was given a 200 mg dose of a drug IV, and 100 mg was eliminated
during the first two hours. If the drug follows first-order elimination kinetics, how much of the drug will remain 6 hours after its administration?

A. None
B. 25 mg
C. 50 mg
D. 75 mg
E. 100 mg

Answer

B. One half of the dose is eliminated in the first two hours so its
elimination half-life equals two hours. With the passage of each half-life
the amount in the body (or in the blood) will decrease to 50% of a former
level. Thus, at 6 hours after administration, three half-lives have passed: 1)
200 mg to 100 mg, 2) 100 mg to 50 mg, and 3) 50 mg to 25 mg.