Which pharmacokinetic principle is demonstrated by a drug with a half-life of 30 minutes, where the concentration halves every 30 minutes?

• A. Steady-state
• B. Half-life
• C. Bioavailability
• D. Clearance

Answer: (B)

The main concept is half-life—the time needed for the drug’s concentration in the body to fall by half. If the concentration halves every 30 minutes, that means the drug has a 30-minute half-life, characteristic of first-order elimination where the rate of loss is proportional to how much drug remains. This also implies predictable drops: after 60 minutes it’s down to a quarter, after 90 minutes to an eighth, and so on. Steady-state refers to the level reached with repeated dosing, not the rate of decline after a single dose. Bioavailability is about how much of the dose reaches circulation, and clearance describes how quickly the body removes the drug, which relates to half-life but isn’t defined by the statement alone.