Pharmacokinetics of metronidazole in horses after intravenous, rectal and oral administration

Metronidazole pharmacokinetics in horses was studied after intravenous (i.v.), rectal (p.r.) and oral (p.o.) administration at 20 mg/kg using a triple crossover study design. Metronidazole mean ±SD half-life was 196 ± 39, 212 ± 30 and 240 ± 65 min after i.v., p.r. and p.o. administration, respectively. The metronidazole clearance was 2.8 (mL/min/kg) and the volume of distribution at steady state was 0.68 L/kg. The pharmacokinetic parameters calculated for metronidazole after administration of the drug by the various routes showed that bioavailability (74 ± 18 vs. 30 ± 9%) and maximum serum concentration (22 ± 8 vs. 9 ± 2 μg/mL) were significantly higher after p.o. administration compared with p.r. administration. There were no significant differences in mean absorption time (45 ± 69 vs. 66 ± 18 min) and the time to reach maximum serum concentration (65 ± 36 vs. 58 ±18 min). The results indicated that p.r. administration of metronidazole to horses, although inferior to p.o. administration in terms of bioavailability, provides an alternative route of administration when p.o. administration cannot be used.