The highest and lowest concentrations of a drug in plasma. Measurement of peak and trough drug levels are used to determine whether an intravenously administered medication is consistently within therapeutic range. The trough is drawn just before a drug is scheduled to be given; the peak is drawn after the drug is administered (30 to 60 min after infusion). These measurements may guide therapy in the use of potentially toxic medications, e.g., aminoglycosides, which can have serious adverse effects if therapeutic levels are exceeded or can fail to work effectively if adequate drug levels are not attained.