Any antibiotic administered orally, intramuscularly, or intravenously can theoretically cause acute diarrhea; however, some antibiotics are more likely than others to upset a horse's gastrointestinal tract.

"Antibiotics are a well-known non-infectious cause of diarrhea in otherwise healthy horses," relays Rodney Belgrave, DVM, MS, Dipl. ACVIM, an internal medicine clinician at Mid-Atlantic Equine Medical Center located in Ringoes, N.J. In addition, oxytetracycline administered in high doses or for prolonged periods to dehydrated horses can cause renal failure.

"This is because antibiotics can kill the normal, healthy microorganisms that reside in the horse's gastrointestinal tract, thereby allowing pathogenic, disease-causing organisms, such as Clostridium, to grow and cause diarrhea," says Belgrave.

Some examples of antibiotics capable of inducing severe diarrhea include orally administered lincomycin, intravenous or intramuscular tetracycline, orally administered penicillin, erythromycin, trimethoprim-sulfa (TMS), metronidazole, doxycycline, and intravenous or intramuscular ceftiofur, enrofloxacin, and gentamicin