Rectal administration of metronidazole with and without rectal evacuation prior to use in horses.

In a randomized crossover design study, 10 adult horses were administered crushed metronidazole tablets rectally at 20 mg/kg. Horses' rectums were either evacuated (E) or not evacuated (NE) of manure prior to the administration of the drug. Serum samples were taken over 24 hr and plasma concentrations were determined via high pressure liquid chromatography. At 15 min post-administration, group E had a significantly higher plasma concentration (p = 0.027), but there were no concentration differences at any other time points. There was large variability in relative bioavailability in the NE group, with a median of 86.7%. Based on our results, there is no advantage to manually evacuating a horse's rectum prior to rectal administration of metronidazole. Further study at higher dosages as well as examination of clinical efficacy is warranted.