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Equine Veterinary Pharmacology

Physiology of Equine GI tract

  • Monogastric with a functioning cecum
  • Proper dental care is essential for digestion
  • Muscles of the esophagus are so strong where they meet the stomach that vomiting or belching is almost impossible

Absorption of Drugs

  • IV = 100% 
  • IM/SubQ = 80-90%
  • Oral absorption can be very erratic 
  • Large pH stomach range (1.5-7.4)
  • Stomach is never empty – can interfere with highly protein bound medications

Drug Excretion: **Use caution administering highly protein bound medications in geriatric horses. These horses can have much lower levels of protein which allows for higher concentrations of “free” drug that can become toxic. (e.g. NSAIDS)**


Colic (Abdominal Pain)

  • Causes: gas, obstruction/impaction, strangulation (tangled intestines), infarction (dead tissue), ulcers
  • Treatment (medical): 
    • Analgesics for pain (flunixin meglumine, detomidine or xylazine)
    • IV fluids for dehydration and shock
    • Lubricants/mineral oil
  • Treating diarrhea associated with Colic (usually bacterial or drug induced)
    • Activated charcoal to absorb endotoxins
    • Kaolin
    • Do NOT use Lomotil (atropine/diphenoxylate) – can have detrimental effects on GI motility 
  • Laxatives
    • Magnesium sulfate – dilute in water
    • Psyllium – dilute in water
    • Dioctyl calcium or sodium sulfosuccinate
  • Motility
    • Metoclopramide
      • Must be diluted
      • Constant rate infusion (CRI) – 0.5mg/mL
    • Lidocaine
      • Given as CRI
  • Equine Plasma
    • Great for treating anterior enteritis, colic and diarrhea
    • Usually need a large volume which can be expensive
      • $150-$225/Liter
  • Dextran 40, Dextran 70, and Hetastarch can be used instead of plasma if equine plasma is not available


Gastric Ulcer

  • Causes: Man-made lesion (stress), NSAIDs, improper feeding, exercise and housing conditions
  • Animal husbandry is required. Horses should be fed quality grass, free choice hay, very little sweet grain, and free choice of water
  • Treatment:
    • Proton pump inhibitors:
      • Omeprazole (GastroGard) paste – only FDA approved product
    • H2 receptor antagonist (off-label)
      • Famotidine IV – high doses can cause colic
    • Sucralfate oral tablets or paste



  • Require antibiotics that have a low volume of distribution and are hydrophilic
  • Aminoglycosides (great gram-negative coverage)
    • Gentamicin: normally diluted – acidic pH (pH 3.5)
    • Amikacin
    • Has a post-kill effect
    • Not for anaerobic infections
  • Secondary anaerobic infections
    • Metronidazole
    • K Penicillin G: used synergistically with metronidazole – administer slowly
      • Penicillin is destroyed by acidic pH – cannot be given using the same catheter as aminoglycosides 
  • Rhodococcus equi
    • Azithromycin – may be added with rifampin to decrease resistant
  • Wound infections 
    • Cefotaxime or Ticarcillin, Zosyn, Bactroban (MRSA)



  • Disease that affects the feet of hooved animals
  • Treatment
    • NSAIDs: 
      • large species variation – need to use a NSAID that is approved for horses
      • Accumulation in horses – can be renal/hepatotoxic
      • Target COX-2 (COX-2 selective agents)
    • Treatment options:
      • Non-selective COX inhibitors
        • Phenylbutazone 
        • Flunixin meglumine (large animal only)
        • Ketoprofen (less renal toxicity)
        • Aspirin (very economical)
      • COX-2 selective
        • Equioxx (firocoxib) – available as a paste, tablet, and injectable
        • Highly COX-2 selective