Omeprazole is a proton pump inhibitor (PPI) that powerfully suppresses stomach acid production. It is one of the most widely used medications in both human and veterinary medicine. In human pharmacies, it is sold over the counter as Prilosec OTC, while in veterinary practice it is the active ingredient in GastroGard and UlcerGard, which are FDA-approved specifically for the treatment and prevention of gastric ulcers in horses. For dogs and cats, veterinarians prescribe omeprazole on an extra-label basis under AMDUCA for acid-related gastrointestinal conditions.
While human OTC omeprazole is available without a prescription, its use in animals should be directed by a veterinarian. In the UK, advising OTC human medicines for animals counts as prescribing under RCVS professional guidance. Even for a familiar medication like omeprazole, appropriate veterinary oversight is important because stomach acid suppression can mask serious underlying conditions.
What Is Omeprazole Used For?
Veterinarians prescribe omeprazole for:
Dogs and Cats
- Gastric ulcers and erosions — treatment and prevention
- Gastroesophageal reflux disease (GERD) — reducing acid exposure to the oesophagus
- Esophagitis — inflammation of the oesophagus from chronic acid reflux
- NSAID-induced gastric damage — protecting the stomach in pets on long-term anti-inflammatory therapy
- Zollinger-Ellison syndrome — rare gastrin-secreting tumours that cause excessive acid production
- Helicobacter-associated gastritis — as part of combination therapy
Horses
- Equine Gastric Ulcer Syndrome (EGUS) — omeprazole is the standard of care for equine gastric ulcers, with FDA-approved formulations (GastroGard for treatment, UlcerGard for prevention)
- Gastric ulcers affect an estimated 60-90% of performance horses and racehorses, making omeprazole one of the most important medications in equine practice
Omeprazole is more potent than H2 blockers like famotidine (Pepcid) and is generally preferred when significant acid suppression is needed.
How Does Omeprazole Work?
Omeprazole works by irreversibly inhibiting the hydrogen-potassium ATPase enzyme system (the proton pump) on the surface of gastric parietal cells. This is the final step in acid production, meaning omeprazole blocks acid secretion regardless of what stimulus triggered it — whether histamine, acetylcholine, or gastrin.
A single dose can reduce gastric acid production by up to 90%. Because it irreversibly binds to the proton pump, the body must synthesize new pumps to restore acid production, which means effects persist even after the drug has been cleared from the bloodstream. Maximum acid suppression typically develops after 3 to 5 days of daily dosing as more proton pumps are inactivated.
Omeprazole is an acid-labile drug, meaning stomach acid can destroy it before it reaches the intestine for absorption. This is why most formulations use an enteric coating. Crushing or splitting enteric-coated tablets or capsules can destroy the protective coating and significantly reduce the drug’s effectiveness.
Dosage
Your veterinarian will determine the correct dose for your pet. The following is general educational information only.
Dogs
| Dog’s Weight | Typical Dose | Frequency |
|---|---|---|
| 10 lbs (4.5 kg) | 5 mg | Once daily |
| 25 lbs (11 kg) | 10 mg | Once daily |
| 50 lbs (23 kg) | 20 mg | Once daily |
| 75 lbs (34 kg) | 20-40 mg | Once daily |
| 100 lbs (45 kg) | 40 mg | Once daily |
The standard dose for dogs is approximately 0.5-1 mg/kg given once daily, typically before the first meal of the day. Higher doses (up to 2 mg/kg) may be used for severe conditions under veterinary direction.
Cats
Cats are typically dosed at 0.5-1 mg/kg once daily. Given the small body weight of most cats, accurately dosing omeprazole can be challenging with human tablet sizes, and compounded formulations may be preferable.
Horses
GastroGard (treatment): 4 mg/kg once daily for 28 days is the FDA-approved protocol. UlcerGard (prevention): 1 mg/kg once daily. These are paste formulations designed specifically for equine use.
How to Give This Medication
- Do not crush, split, or chew enteric-coated capsules or tablets. The enteric coating is essential for protecting the drug from stomach acid.
- Give on an empty stomach, ideally 30-60 minutes before the first meal, for optimal absorption.
- For horses, GastroGard/UlcerGard paste is given directly onto the back of the tongue using an oral dosing syringe.
- If your veterinarian prescribes a compounded formulation (suspension or smaller capsules), follow their specific administration instructions.
- If you miss a dose, give it as soon as you remember. If it is close to the next scheduled dose, skip the missed dose.
- Do not stop abruptly after prolonged use. Your veterinarian may recommend a gradual taper to avoid rebound acid hypersecretion.
Side Effects
| Common Side Effects | Less Common / Serious Side Effects (Contact Your Vet) |
|---|---|
| Generally well tolerated | Diarrhoea |
| Mild nausea when starting | Vomiting |
| Flatulence | Decreased appetite |
| Changes in gut microbiome with prolonged use | |
| Possible interference with calcium and magnesium absorption (long-term) | |
| Possible increased susceptibility to GI infections (long-term acid suppression) |
Omeprazole is well tolerated at standard doses in dogs, cats, and horses. Long-term use (months to years) may carry risks related to chronic acid suppression, including altered nutrient absorption and changes to the gut bacterial environment. Your veterinarian will weigh these considerations against the benefits of ongoing treatment.
Drug Interactions
Inform your veterinarian about all medications your pet is taking. Significant interactions include:
- Ketoconazole and itraconazole — require stomach acid for absorption; omeprazole may dramatically reduce their effectiveness
- Clopidogrel (Plavix) — omeprazole may reduce clopidogrel’s antiplatelet activity in some patients
- Sucralfate — administer at least 30 minutes apart; sucralfate requires an acidic environment
- Phenobarbital — potential for altered absorption
- Methotrexate — omeprazole may increase methotrexate levels
Contraindications
Omeprazole should be used with caution or avoided in:
- Pets with known hypersensitivity to omeprazole or other PPIs
- Pets with liver disease — omeprazole is metabolised by the liver
- Pregnancy — safety not fully established in pregnant animals
Storage
- Store capsules and tablets at room temperature, 68-77°F (20-25°C), protected from moisture and light.
- GastroGard/UlcerGard paste: store at controlled room temperature; do not freeze.
- Keep out of reach of children and pets.
Cost & Availability
Omeprazole is available in both human OTC formulations and veterinary-specific products, with significant price differences.
- Human OTC omeprazole: Generic 20 mg capsules cost approximately $8-$20 for a 28-42 day supply at most pharmacies
- GastroGard (equine): Significantly more expensive — approximately $30-$40 per day for a full-size horse at treatment doses, though generic equine omeprazole paste is now available at lower cost
- UlcerGard (equine prevention): Less expensive than GastroGard but still considerably more than human generic omeprazole
- Veterinary-dispensed for dogs/cats: Often more expensive than purchasing OTC at a human pharmacy
- GoodRx (goodrx.com/pets) can help find the best prices at participating pharmacies
- Costco pharmacy does not require a membership for pharmacy purchases
For dogs and cats, many veterinarians write prescriptions that can be filled with human generic omeprazole at substantial cost savings compared to veterinary-dispensed product. The equine market has historically been more expensive, though generic equine formulations have improved affordability.
Frequently Asked Questions
Can I give my dog over-the-counter Prilosec? Human OTC omeprazole (Prilosec OTC) contains the same active ingredient used in veterinary practice for dogs. However, you should only give it under veterinary direction for appropriate dosing and to ensure it is treating the right condition. Vomiting in dogs can signal many serious problems that acid suppression alone will not address.
How long does omeprazole take to work? Acid reduction begins within hours of the first dose, but maximum effectiveness develops over 3 to 5 days of daily dosing. For gastric ulcer healing, treatment courses typically last 2 to 8 weeks depending on severity.
Is omeprazole better than famotidine for dogs? Omeprazole provides more potent and sustained acid suppression than famotidine. For confirmed gastric ulcers, severe reflux, or NSAID-protective therapy, omeprazole is generally preferred. Famotidine has a faster onset and fewer drug interactions, making it suitable for milder conditions. Your veterinarian will recommend the appropriate option.
Why is GastroGard so much more expensive than human omeprazole? GastroGard is an FDA-approved veterinary product specifically formulated as a paste for equine administration at the higher doses horses require. The regulatory approval process, specialized formulation, and smaller market contribute to the higher price. Some horse owners and veterinarians use human omeprazole formulations as a cost-saving measure, though bioavailability may differ between formulations. Discuss options with your equine veterinarian.
Can I use omeprazole for my cat? Yes, omeprazole is used in cats under veterinary direction. Accurate dosing can be challenging due to the small body size of most cats and the need to avoid crushing enteric-coated formulations. Your veterinarian may recommend a compounded formulation for easier and more accurate dosing.