What is the trough drug level?

• A. The lowest blood level of a drug
• B. The highest blood level
• C. The average blood level
• D. The time to onset

Answer: (A)

The trough drug level is the lowest concentration of a drug in the blood within a dosing interval, measured right before the next dose is given. This point is monitored to ensure the drug remains above the minimum effective level and stays below toxic levels, guiding dose adjustments if needed. It contrasts with the peak level, which is the highest concentration reached after a dose, and with the time to onset, which refers to when the drug starts to work rather than its concentration in the blood.