The trough level is defined as what?

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

Answer: (B)

The trough level is the lowest concentration of a drug in the blood during a dosing interval, measured just before the next scheduled dose. This measurement helps ensure the drug remains within the therapeutic window, avoiding levels that are too low to be effective or high enough to cause toxicity. The highest concentration after dosing is the peak level, which serves a different purpose, while the therapeutic level refers to the range in which the drug is expected to work. By monitoring trough levels, clinicians can adjust dose or dosing interval to maintain adequate exposure between doses.