Effect of norfloxacin on human oropharyngeal and colonic microflora and multiple-dose pharmacokinetics

10 healthy volunteers received 200 mg norfloxacin orally every 12 h for 7 days. Saliva, throat and faecal specimens were collected days 0, 3, 5, 7, 14 and 21 to study the effect of norfloxacin on the normal microflora. The concentrations of norfloxacin in serum, urine, saliva and faeces were determined by a microbiological method and all samples except faeces were also assayed by high-pressure liquid chromatography (HPLC). The pharmacokinetics of norfloxacin were studied on day 3. The mean peak serum concentration (+/- SD) attained after 0.75-1.0 h was 0.75 +/- 0.15 mg/l measured by HPLC, and the mean terminal serum half-life was 4.2 +/- 0.6 h. The mean cumulative urinary elimination was 29% during 12 h after dosing. There was no significant difference between values obtained by microbiological assay and by HPLC. The saliva concentration was approximately 30% of the serum levels 1.0-1.5 h after administration. No accumulation in faeces was found during the administration period, and mean concentrations were 940 mg/kg. The changes in the oropharyngeal flora were minor and only branhamella were affected. In the colonic flora, the number of enterobacteria was strongly depressed while the anaerobic microflora was only slightly affected. Two weeks after the administration period, both the oropharyngeal and colonic microflora had returned to normal.