Serum digoxin concentrations in dogs before and during concomitant administration of furosemide

Healthy dogs were treated once a day for 16 days with a liquid, oral dosage form of digoxin (0.022 mg/kg). From day 9 to 16 they were also injected intramuscularly with furosemide (4.4 mg/kg). Serum digoxin was measured by a radioimmunoassay technique. Eight hours after the eighth dose of digoxin had been administered, serum digoxin concentration was in the accepted therapeutic range. After 8 days of concomitant administration of digoxin and furosemide, serum digoxin concentration was found to be in the accepted moderate‐to‐severe toxic range. Clinical signs of digitalis toxicosis were consistently observed during the combined digoxin‐plus‐furosemide treatment period. There was no significant ( P >0.05) change in the serum concentrations of potassium, sodium, or in osmolality during digoxin treatment alone. Serum creatinine concentrations remained within the accepted normal range for dogs. Serum sodium concentration was significantly ( P <0.05) lower during combined digoxin‐plus‐furosemide treatment when compared to digoxin treatment only. Results indicate that an interaction between digoxin and furosemide occurred which led to significantly ( P <0.05) higher concentrations of serum digoxin during combined digoxin and furosemide treatment.