Serum digoxin concentrations in dogs before, during, and after concomitant treatment with phenobarbital

Healthy dogs were treated once a day for 26 days with a liquid, oral dosage form of digoxin (0.022 mg/kg). From day 10 to 24 they were also given phenobarbital (12 mg/kg, sid, orally). Serum digoxin concentration was measured by a radioimmunoassay technique. Eight hours after the 2nd daily dose of digoxin had been administered, serum digoxin concentration was in the accepted therapeutic range. After 3 days of concomitant administration of digoxin and phenobarbital, serum digoxin concentration began to increase. Beginning on day 15 until day 24 (last day of combined digoxin and phenobarbital treatment), a significant (P less than 0.05) increase in serum digoxin concentrations occurred. Blood urea nitrogen values remained within the normal limits throughout the study. There was no significant (P greater than 0.05) difference between base-line heart rate and P-R intervals and the values recorded during the investigation.