Concentrations of phenobarbital in epileptic dogs on phenobarbital or primidone therapy

Serum concentrations of phenobarbital were measured in 223 epileptic dogs treated with phenobarbital and in 55 epileptic dogs treated with primidone. Regression analysis showed for both drugs a significant correlation between serum concentrations and dosages related to body weight (phenobarbital: r = 0.83 Spearman correlation, p < 0.01, primidone: r = 0.86 Pearson correlation p < 0.01). However, the serum concentrations scattered considerably around the regression line. The age of the dogs had no influence on the concentration of phenobarbital. When treated for at least 6 months with constant phenobarbital dosages, 81 out of 90 dogs showed phenobarbital concentrations between 11.8 and 38.9 mu g/ml. The corresponding dosages were between 2.6 and 15.4 mg/kg and day. Treated with primidone about 90% of the dogs (n = 32) had phenobarbital concentrations between 1.5 and 44.4 mu g/ml, the dosages varied between 8.7 and 65.1 mg/kg and day. The serum concentration of phenobarbital can be roughly estimated from the phenobarbital and primidone dosages related to body weight. However, because of the high variability the optimal dosage for therapy can only be found by determining the phenobarbital serum concentration. In dogs who are not seizure free phenobarbital concentrations in the upper therapeutic range should be aimed for.