These drugs are intended to fight infection by killing bacteria that have entered the horse’s
bloodstream. Unfortunately, they also kill beneficial bacteria in the digestive tract. An oral antibiotic often used in horses is Sulfamethoxazole and trimethoprim (SMZ). Sulfamethoxazole works as an antibiotic by inhibiting bacterial synthesis of dihydrolic acid by competing with para-aminobenzoic acid (PABA). Trimethoprim blocks the production of tetrahydrofolic acid from dihydrofolic acid by binding to and reversibly inhibiting the required enzyme, dihydrofolate reductase. It is not known what effect chronic SMZ administration has on gut function in horses, but it has been reported to cause diarrhea and colic in humans.