Serum concentration of penicillin in the horse after repeated intramuscular injections of procaine penicillin G alone or in combination with benzathine penicillin and/or phenylbutazone.

Twenty-one adult horses were randomly assigned into 7 groups of 3 and were treated for 5 days with procaine penicillin G, benzathine penicillin , or phenylbutazone in various combinations and dosage schedules. Serum concentration of penicillin was measured serially over a 7-day period. The highest mean peak serum concentration was 2.06 micrograms/ml. Comparable peak values were seen 2 to 4 hours after administration of 22,000 IU of procaine penicillin G/kg of body weight given once or twice daily. A minimum serum concentration of 0.25 micrograms/ml was selected as adequate for efficacy against organisms that can be considered sensitive to penicillin. Once daily administration of procaine penicillin G and, although peak concentrations were lower, some regimens of procaine and benzathine penicillin G did maintain this concentration. Concurrent phenylbutazone administration did not affect serum concentration of penicillin appreciably.