Omeprazole — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	0.5-1 mg/kg	q12h (every 12 hours) or q24h (every 24 hours)	Duration: 4-8 weeks for ulcer healing; long-term for chronic conditions Notes: Administer on an empty stomach, 30-60 minutes before a meal. For severe ulcers, use q12h initially, then reduce to q24h for maintenance.
Cat	PO	0.5-1 mg/kg	q12h or q24h	Duration: 4-8 weeks; long-term for chronic conditions Notes: Cats may require q12h for adequate acid suppression. Use compounded formulations if needed.
Horse	PO	1-4 mg/kg	q24h	Duration: 4-6 weeks for ulcer healing; maintenance at lower dose Notes: Use buffered oral paste or suspension. Administer on an empty stomach. For severe EGUS, use 4 mg/kg q24h for 4 weeks, then reduce to 1-2 mg/kg for maintenance.
Cattle	PO	1-2 mg/kg	q24h	Duration: 3-7 days or as needed Notes: Limited data; use with caution. May be used for abomasal ulcers.
Small Ruminants (sheep/goats)	PO	1-2 mg/kg	q24h	Duration: 3-7 days or as needed Notes: Limited data; use with caution.
Rabbit	PO	0.5-1 mg/kg	q24h	Duration: 7-14 days or as needed Notes: Use with supportive care for gastric stasis.
Birds/Poultry	PO	0.5-1 mg/kg	q24h	Duration: 7-14 days or as needed Notes: Limited data; use with caution.
Exotic/Other (ferrets, reptiles)	PO	0.5-1 mg/kg	q24h	Duration: 7-14 days or as needed Notes: Limited data; use with caution.

Clinical Indications & Species Uses

General Indications

Gastric acid suppression
Treatment and prevention of gastric ulcers
Esophagitis
Gastroesophageal reflux disease

Dog (Canine)

Gastric ulcers
Duodenal ulcers
Esophagitis
Gastroesophageal reflux disease (GERD)
Mast cell tumor degranulation prophylaxis (with H2 blockers)
Stress-related gastric erosions
Helicobacter-associated gastritis (adjunctive therapy)

Cat (Feline)

Gastric ulcers
Esophagitis
Gastroesophageal reflux disease (GERD)
Chronic vomiting associated with gastritis
Stress-related gastric erosions

Horse (Equine)

Equine gastric ulcer syndrome (EGUS)
Glandular gastric ulceration
Squamous gastric ulceration
Prevention of exercise-induced gastric ulcers
NSAID-induced gastric ulceration

Cattle (Bovine)

Abomasal ulcers
Gastrointestinal bleeding
Prevention of stress-related abomasal ulcers
Acidosis-related abomasitis (supportive)

Small Ruminants (Sheep / Goat)

Abomasal ulcers
Gastroesophageal reflux
Stress-related gastric ulcers

Rabbit & Small Mammals

Gastric ulcers
Gastric stasis (adjunctive)
Trichobezoar management (adjunctive)

Avian & Poultry

Gastric ulcers
Gastroesophageal reflux
Proventricular ulceration (supportive)

Exotic & Other Species

Gastric ulcers in ferrets
Gastroesophageal reflux in reptiles (limited data)
Stress ulcer prophylaxis in exotic mammals

Pharmacology & Mechanism of Action

Drug Class: Proton Pump Inhibitor (PPI) | Pharmacological Group: Substituted benzimidazole

Mechanism of Action: Omeprazole is a weak base that accumulates in the acidic environment of gastric parietal cell canaliculi. It is converted to the active sulfenamide form, which forms covalent disulfide bonds with the H+/K+-ATPase enzyme (the proton pump) on the luminal surface of the parietal cell membrane. This irreversibly inhibits the enzyme, blocking the final step of gastric acid secretion. Because the inhibition is irreversible, acid secretion resumes only after new pump molecules are synthesized, providing a prolonged duration of action (up to 24-48 hours in most species).

Pharmacodynamics: Omeprazole dose-dependently suppresses basal and stimulated gastric acid secretion. It raises intragastric pH, which promotes healing of gastric and duodenal ulcers, reduces gastric irritation, and helps prevent stress-related mucosal damage. It also reduces gastric acid volume and pepsin activity. In horses, it increases gastric pH above 4 for up to 24 hours after oral administration. In dogs and cats, it effectively controls gastric hyperacidity and is used for esophagitis and ulcer management.

⚡ Pharmacokinetics Summary

Absorption: Omeprazole is a weak base and is acid-labile; therefore, oral formulations are enteric-coated or formulated as buffered suspensions to protect the drug from gastric degradation. Absorption occurs in the proximal small intestine. Food can reduce the rate and extent of absorption, so it is recommended to administer on an empty stomach, ideally 30-60 minutes before a meal. In dogs, oral bioavailability is approximately 40-50% for the enteric-coated formulation, but may be lower in some individuals. In horses, oral bioavailability is variable (around 10-20%) for the paste formulation, but higher when given as a buffered suspension.

Distribution: Omeprazole is widely distributed in body tissues. It crosses the placenta and is distributed into milk. It has a volume of distribution of about 0.3-0.5 L/kg in dogs. It is extensively bound to plasma proteins (approximately 95% in humans, similar in animals).

Metabolism: Omeprazole is extensively metabolized in the liver by cytochrome P450 enzymes (CYP2C19 and CYP3A4 in humans; similar enzymes in animals). The metabolites are inactive. Hepatic metabolism is the primary route of elimination.

Excretion: Metabolites are primarily excreted in the urine (about 77%) and the remainder in feces via bile. Less than 1% of the parent drug is excreted unchanged in urine. In dogs, the elimination half-life is about 1-2 hours, but the pharmacodynamic effect lasts much longer due to irreversible enzyme inhibition.

Half-Life: Dogs: 1-2 hours; Cats: 1-2 hours; Horses: 1-2 hours (but effect lasts 24 hours); Cattle: 1-2 hours

Bioavailability: Dogs: ~40-50% (oral enteric-coated); Cats: ~50% (oral); Horses: ~10-20% (oral paste), higher with buffered suspension; Cattle: ~20-30% (oral)

Protein Binding: Approximately 95% in dogs and cats; 90-95% in horses

Available Formulations & Strengths

Oral Tablet (enteric-coated) 10 mg, 20 mg, 40 mg (PO)

Oral Capsule (delayed-release) 10 mg, 20 mg, 40 mg (PO)

Oral Paste (buffered) 370 mg/g (equine) (PO)

Oral Suspension (compounded) 2 mg/mL, 10 mg/mL (PO)

Injectable Solution (IV) 40 mg/vial (for reconstitution) (IV)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to omeprazole or other benzimidazoles
Concurrent use with atazanavir or nelfinavir (antiretroviral drugs) - not typically used in veterinary medicine but relevant in exotic species
Use in animals with severe hepatic impairment (caution)
Do not use in animals with known gastric malignancy (may mask symptoms)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic disease; dose adjustment may be needed.
Long-term use may increase the risk of gastric polyps or hypergastrinemia.
May interfere with the absorption of drugs that require an acidic gastric environment (e.g., ketoconazole, itraconazole, iron supplements).
In horses, ensure proper administration technique to avoid aspiration of paste.
In food animals, observe withdrawal times; extra-label use requires veterinary oversight.
Pregnancy and lactation: Use only if clearly needed; safety not fully established.
Do not crush or chew enteric-coated tablets; they must be swallowed whole.
Compounded suspensions should be used within the expiration date and stored appropriately.

Adverse Effects & Reactions

Common:

Vomiting
Diarrhea
Loss of appetite
Flatulence
Abdominal pain
Salivation (especially in cats)

Serious / Severe:

Hypersensitivity reactions (angioedema, anaphylaxis)
Hepatotoxicity (rare)
Interstitial nephritis (rare)
Bone marrow suppression (rare)
Hypergastrinemia leading to gastric polyps (with long-term use)

Rare:

Skin rash
Fever
Thrombocytopenia
Leukopenia
Gynecomastia (in males with long-term use)
Hypomagnesemia (with prolonged use)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Ketoconazole, Itraconazole, other azole antifungals	Reduced absorption of azole antifungals due to increased gastric pH.	Moderate
Diazepam, phenytoin, warfarin (CYP2C19 substrates)	Omeprazole may inhibit CYP2C19, increasing plasma levels of these drugs.	Moderate
Clopidogrel	Omeprazole may reduce the activation of clopidogrel, decreasing its antiplatelet effect.	High
Iron supplements	Reduced iron absorption due to increased gastric pH.	Mild
Sucralfate	Sucralfate may bind to omeprazole and reduce its absorption; separate administration by at least 30 minutes.	Moderate
Digoxin	Increased digoxin absorption due to reduced gastric acid degradation.	Mild
Methotrexate	Omeprazole may reduce renal clearance of methotrexate, increasing toxicity risk.	High

Overdose & Toxicity Management

Signs of Toxicity:

Vomiting
Diarrhea
Abdominal pain
Drowsiness
Confusion
Tachycardia
Hypotension (in severe cases)

Emergency Treatment Protocol: Treatment is primarily symptomatic and supportive. Induce vomiting if ingestion is recent and the animal is conscious (do not induce in horses). Administer activated charcoal to reduce absorption. Monitor vital signs and provide fluid therapy if needed. There is no specific antidote; omeprazole is highly protein-bound, so dialysis is not effective.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 3 days

Withdrawal times are not officially established for omeprazole in food animals. The values provided are conservative estimates based on pharmacokinetic data and common practice. Always consult the label or a veterinary pharmacologist for specific guidance. Extra-label use in food animals requires a valid veterinary-client-patient relationship and observance of extended withdrawal times.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F); excursions permitted between 15-30°C (59-86°F).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from moisture and light. Keep in original container. Compounded suspensions should be refrigerated and used within 30 days unless otherwise specified.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: FDA-approved for use in dogs (GastroGard) and horses (GastroGard, UlcerGard). Human formulations are available and may be used extra-label.

Extra-Label (Off-Label) Use: In the US, omeprazole is approved for use in dogs and horses. Extra-label use in other species is permitted under AMDUCA (Animal Medicinal Drug Use Clarification Act) with a valid veterinary-client-patient relationship. For food animals, a withdrawal time must be established by the veterinarian.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Omeprazole is a potent and long-acting proton pump inhibitor used primarily for the treatment and prevention of gastric ulcers and esophagitis in veterinary patients. In dogs and cats, it is often preferred over H2 blockers for severe acid suppression. In horses, it is the drug of choice for equine gastric ulcer syndrome. Key clinical points: (1) Administer on an empty stomach for optimal absorption; (2) For horses, use the buffered paste or suspension formulation; (3) In dogs and cats, q12h dosing may be necessary for complete acid suppression; (4) Long-term use may require monitoring for hypergastrinemia and potential gastric polyps; (5) Consider drug interactions, especially with drugs that require an acidic environment for absorption; (6) In food animals, adhere to withdrawal times and extra-label regulations. Always tailor therapy to the individual patient and underlying condition.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)