Effect of some drugs on penicillin half‐life in blood

The half‐life (T /2) of penicillin in blood was determined in 61 patients. An inverse relationship of penicillin T/2 and endogenous creatinine clearance was found. In a group of elderly patients with normal serum creatinine, penicillin T/2 was prolonged due to age‐dependent decrease in renal function. The effect of several drugs on the active renal tubular transport mechanism of penicillin was studied. Probenecid increased penicillin T/2 from an average of 40.4 to 104.3 minutes. Penicillin T/2 in the younger patients using probenecid was of the same order as penicillin T/2 in elderly individuals not using probenecid. Phenylbutazone increased penicillin T/2 almost as much as did probenecid. Sulfinpyrazone, acetylsalicylic acid, indomethacin, and sulfaphenazole in therapeutic doses increased penicillin T /2 to a smaller degree. There was no significant change in penicillin T /2 after therapeutic doses of chlorothiazide, sulfamethizole, and sulfamethoxypyridazine.