The pharmacology of furosemide in the horse: V. the duration of reduction of urinary concentration of drugs

The administration of furosemide to horses in IVdoses of 0.5 mg/ kg or less reduced drug concentrations in urine for less than 4 hours. The most prolonged reduction observed was that of the glucuronide metabolite of morphine, which required three hours post-dosing to return to control. Urinary concentrations of phenylbutazone were not significantly different from control by two hours post-dosing, while urinary concentrations of fentanyl appeared to return to normal within about two and one-half hours of dosing. Other experiments showed that blood levels of morphine were not significantly reduced by furosemide treatments. Studies on the urinary concentrations of “Phenylbutazone and its metabolites” in a population of racing horses, some of which had been pretreated with furosemide pre-race, showed that the apparent dilution effect observed in actual practice was about 50%. These experiments therefore suggest that under practical conditions the amount of interference with drug detection by furosemide is small and likely negligible if the dose commonly used in the treatment of epistaxis is given three or more hours prior to sample taking.