Figure 1: Peak and trough serum Phenobarbital levels in normal dogs (n=5) given oral Phenobarbital at 4 mg/kg every 12 hours. Levels are expressed in μg/ml for the total Phenobarbital concentrations taken before each medication (trough) period and 2 hours after medication (peak). Bars represent SD of the mean value. There was a dip in the total Phenobarbital levels that occurred around the 4-6 day when patients were observed to exhibit increased thirst (PD) and urination (PU). In addition, a transient sedation was noted around day 5 which continued through day 7.