Twelve-hourly dosage schedule for oral and intravenous metronidazole

Paired samples of blood were examined from 48 patients undergoing major surgery who were receiving metronidazole at a 12-hourly dosage interval, as part of a regimen for prophylaxis or therapy. Twenty-five patients were given 400 mg metronidazole orally twice a day and 23 received 500 mg metronidazole iv twice daily. Assay of the samples demonstrated trough serum concentrations of metronidazole of 3-11 mg/l (mean 5.5 mg/l) and 2-15 mg/l (mean 6.7 mg/l) with the oral and intravenous routes, respectively. The corresponding peak serum concentrations were 10-26 mg/l (mean 17.4 mg/l) and 13-28 mg/l (mean 23.6 mg/l). The trough concentrations are well in excess of the MICs for the majority of obligate anaerobes and thus the 12-hourly regimen achieves and maintains therapeutic serum concentrations of metronidazole.