A nurse is administering a medication with a half-life of 6 hours. If the initial dose is 800 mg, how much of the medication will remain in the patient's system after 18 hours?

A. 300mg is incorrect. B. 400mg is incorrect C. 200mg is incorrect D. The half-life of a drug is the time it takes for the plasma concentration of a drug to decrease by 50%. If the half-life is 6 hours, the amount of drug in the body will halve every 6 hours. Initial dose: 800 mg After 6 hours (1st half-life): 800 mg ÷ 2 = 400 mg After 12 hours (2nd half-life): 400 mg ÷ 2 = 200 mg After 18 hours (3rd half-life): 200 mg ÷ 2 = 100 mg After 18 hours (three half-lives), 100 mg of the medication will remain in the patient's system.