If a patient on edoxaban stops prior to surgery and the half-life is 12 hours, taking one more tablet and then stopping will take how long to reach 25% of the initial plasma concentration?

• A. 12 hours
• B. 24 hours
• C. 6 hours
• D. 48 hours

Answer: (B)

The main idea is that drug levels fall by a constant proportion—the half-life—after you stop dosing. Edoxaban has a 12-hour half-life, so after a dose is taken and then stopped, the plasma concentration halves every 12 hours. If one more tablet is taken and then you stop, the concentration peaks after that dose and then declines. To reach 25% of the level just after that last dose, you need two half-lives: 12 hours to drop to 50%, and another 12 hours to drop to 25%. So it takes 24 hours in total.