After 4 hours: 60 ng/mL, After 8 hours: 30 ng/mL, After 12 hours: 15 ng/mL, After 16 hours: 7.5 ng/mL.
The half-life of a drug is the time it takes for the concentration of the drug in the body to reduce by half. In this case, the half-life of the drug is 4 hours. When a patient discontinues taking the medication, the concentration of the drug decreases exponentially over time.
The initial concentration is given as 120 ng/mL. After the first 4 hours, which is one half-life, the concentration reduces to 60 ng/mL. Subsequent half-lives continue to halve the concentration. After 8 hours, it becomes 30 ng/mL, after 12 hours it's 15 ng/mL, after 16 hours it's 7.5 ng/mL.
This exponential decrease reflects the nature of drug elimination from the body, with each half-life reducing the concentration by half. The rounded values in the table represent the decreasing drug concentrations at different time intervals after the patient stops taking the medication.