Omeprazole
Omeprazole is a highly effective **proton pump inhibitor (PPI)** widely used in veterinary medicine for the treatment and prevention of gastroduodenal ulcers and erosions. * **Superior Efficacy:** It is generally considered superior to H2-receptor antagonists (such as famotidine) for raising gastric pH and preventing exercise-induced gastritis, as demonstrated in racing Alaskan sled dogs. * **Equine Use:** An oral paste formulation is specifically labeled and highly effective for the treatment and prevention of gastric ulcers in horses (EGUS). * **Broad Application:** It is utilized across multiple species, including dogs, cats, ferrets, and swine, to manage conditions like uremic gastropathy, esophagitis, Helicobacter infections, and NSAID/corticosteroid-induced ulceration. > **Clinical Pearl:** Because omeprazole requires an acidic environment for activation but is degraded by stomach acid before absorption, human oral formulations are enteric-coated. These must not be crushed or chewed, as this will destroy the drug's bioavailability.
Mechanism: Omeprazole is a **prodrug** that acts as a substituted benzimidazole gastric acid pump inhibitor. 1. **Absorption & Distribution:** After absorption, it diffuses from the blood into the highly acidic secretory canaliculi of the gastric **parietal cells**. 2. **Activation:** In this acidic environment (pH < 3), it is protonated and rearranged into its active form, a **sulphenamide derivative**. 3. **Irreversible Binding:** The active sulphenamide binds **irreversibly** via disulfide bonds to the **Hโบ/Kโบ ATPase enzyme** (the proton pump) at the secretory surface. 4. **Acid Suppression:** This blocks the final common pathway of gastric acid secretion, inhibiting the transport of hydrogen ions into the stomach lumen during both basal and stimulated conditions. > **Pharmacological Note:** Because the binding is irreversible, acid secretion only resumes when new Hโบ/Kโบ ATPase enzymes are synthesized. This explains why its duration of action (24-72 hours) far outlasts its short plasma half-life (~1 hour). Omeprazole also inhibits the hepatic **cytochrome P-450** mixed-function oxidase system, which can lead to drug interactions.
Dosing by species
- Adjunctive treatment of esophagitis or gastric ulcers ยท 0.5-1 mg/kg PO q24h ยท PO ยท q24h
- GI ulcer management/prevention ยท 0.7-1.5 mg/kg PO q12-24h ยท PO ยท q12h-q24h
- Adjunctive treatment of uremic gastropathy ยท 0.7 mg/kg PO q24h ยท PO ยท q24h ยท Dosage may need to be modified in moderate or severe renal failure.
- Short-term treatment of gastroenteritis ยท 0.7 mg/kg PO q24h ยท PO ยท q24h ยท Short-term
- Treatment of gastric ulcers ยท 4 mg/kg PO once daily for 4 weeks; to prevent recurrence treat for at least another 4 weeks at 2 mg/kg PO once daily ยท PO ยท q24h ยท 8 weeks total ยท ARCI UCGFS Class 5 Drug
- Foals: Preventative dose ยท 1 mg/kg PO q24h ยท PO ยท q24h ยท There has been a recent shift to not administering prophylactic antiulcer medication routinely to sick foals.
- Foals: Treatment dose ยท 4 mg/kg PO q24h ยท PO ยท q24h ยท Will reduce gastric pH in a few hours.
- Treatment or prophylaxis of gastric ulcers in foals ยท 4 mg/kg PO once daily for treatment, 1-2 mg/kg PO once daily for prophylaxis ยท PO ยท q24h
- Foals: Gastric ulcers ยท 4 mg/kg PO q24h ยท PO ยท q24h ยท Commonly foals will be started on ranitidine (1.5 mg/kg IV q8h; 6.6 mg/kg PO q8h) and omeprazole together.
Routes of administration
Contraindications
- Known hypersensitivity to omeprazole
Adverse effects
- GI distress (anorexia, colic, nausea, vomiting, flatulence, diarrhea)
- Hematologic abnormalities (rare)
- Urinary tract infections
- Proteinuria
- CNS disturbances
- Urticaria (rare in horses)
Drug interactions
- Benzodiazepines (e.g., diazepam) ยท Omeprazole may potentially alter benzodiazepine metabolism and prolong CNS effects
- Clarithromycin ยท Increased levels of omeprazole, clarithromycin and 14-hydroxyclarithromycin are possible
- Cyanocobalamin (oral) ยท Omeprazole may decrease oral absorption
- Cyclosporine ยท Omeprazole may reduce cyclosporine metabolism
- Ketoconazole, Itraconazole, Iron, Ampicillin esters ยท Omeprazole increases gastric pH, which may decrease the absorption of these drugs that require an acidic environment
- Warfarin ยท Omeprazole may increase anticoagulant effect
Monitoring
- Clinical efficacy (resolution of clinical signs of ulcers/esophagitis)
- Adverse effects (GI distress)
- Liver enzymes (may increase)
- Serum gastrin levels (will increase early in therapy)
Overdose
The LD50 in rats after oral administration is reportedly >4 grams/kg. Humans have tolerated oral dosages of 360 mg/day without significant toxicity. Should a massive overdose occur, treat symptomatically and supportively. Due to its wide safety margin, acute toxicity from accidental ingestion is generally mild.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.