Pharmacologic interactions among chloramphenicol, phenytoin and phenobarbital.

Bioactive chloramphenicol was measured in the sera of 34 infants and children receiving intravenous chloramphenicol succinate (25 mg/kg/dose) as therapy for meningitis, pneumonia, brain abscess or Rocky Mountain spotted fever. In 17 children receiving chloramphenicol succinate alone, mean peak and trough serum concentrations of chloramphenicol were 25.3 and 13.4 micrograms/ml, respectively. Concurrent administration of chloramphenicol succinate and phenobarbital in six patients resulted in reduced peak and trough concentrations of chloramphenicol of 16.6 and 7.5 micrograms/ml, respectively (P less than 0.05). Concurrent administration of chloramphenicol succinate and phenytoin in six patients resulted in an elevated mean peak serum concentration of chloramphenicol of 41.7 micrograms/ml (P less than 0.05). Five other patients treated with phenytoin had indeterminate serum chloramphenicol half-lives with chloramphenicol concentrations of 29 to 90 micrograms/ml. Potentially toxic chloramphenicol concentrations occurred in 9 of 17 controls, 0 of 6 patients who had received phenobarbital (P = 0.001) and 11 of 11 patients who had received phenytoin (P = 0.008). Patients receiving combined chloramphenicol succinate and anticonvulsant therapy require monitoring of serum concentrations of drug and adjustments in the dose of chloramphenicol succinate administered.