How would you use trough levels to adjust dosing for a drug with a narrow therapeutic window?

• A. Decrease dose
• B. Do nothing
• C. Increase dose or shorten interval
• D. Stop therapy

Answer: (C)

For drugs with a narrow therapeutic window, trough levels are used to fine-tune dosing so concentrations stay within the safe and effective range. The trough is the lowest concentration right before the next dose, and at steady state it reflects overall exposure over the dosing interval. If the trough is below the therapeutic range, the patient isn’t getting enough drug activity, so you raise exposure by increasing the dose or shortening the dosing interval. Increasing the dose raises both peak and trough, while shortening the interval delivers more frequent dosing, raising the trough and the overall average concentration. If trough were too high, you’d do the opposite—decrease the dose or lengthen the interval. When troughs are in range, you maintain the regimen.