Omeprazole is used to treat gastric ulcers in horses and esophageal problems by decreasing the production of stomach acid. It is potentially useful in the treatment of both gastroduodenal ulcer disease and prevention or treatment of gastric erosions caused by ulcerogenic drugs (eg, aspirin, NSAIDs).¹

In one study, researchers used six horses treated with either 1 mg/kg or 4 mg/kg of omeprazole while receiving either a high-grain/low-fiber diet or a diet comprised of free-choice hay. The “area under the curve” (AUC), which is a measure of how much of the body is exposed to the drug after administration, was higher in horses treated with 4 mg/kg rather than 1 mg/kg of omeprazole. Although the AUC was higher in horses treated with omeprazole and fed a high-grain/low-fiber diet rather than hay, the difference between the two groups was not statistically significant. Further, there was a great deal of variability observed in the AUCs in the high-grain/low-fiber diet, suggesting that perhaps even though absorption of omeprazole in horses fed hay is lower than the high-grain/low-fiber diet, the absorption is more predictable and therefore desirable.²

Omeprazole: Pharmacology & Pharmacokinetics

Omeprazole is a substituted benzimidazole gastric acid pump inhibitor. In an acidic environment, omeprazole is activated to a sulfenamide derivative that binds irreversibly at the secretory surface of parietal cells to the enzyme H+/K+ ATPase, where it inhibits the transport of hydrogen ions into the stomach by 90% to 99%. Omeprazole can reduce acid secretion during both basal and stimulated conditions. A lag time between administration and efficacy may be observed.¹

Pharmacokinetically, omeprazole’s effects on acid secretion are independent of its drug levels. Omeprazole is rapidly absorbed from the gut; the human commercial product is in an enteric-coated granule form, as the drug is rapidly degraded by acid. The equine paste is not enteric coated. In humans, peak serum levels can occur within 0.5 to 3.5 hours, and onset of action can occur within an hour. The FDA-approved equine oral paste is ≈10% bioavailable due to a significant first-pass effect. Food reduces omeprazole absorption on horse by as much as 67%.¹

Oral paste products containing omeprazole are labeled for use in horses and foals 4 weeks of age and older. The drug effectively treats^3,4 and prevents^5,6 gastric ulcers, and is considered the drug of choice of equine gastric ulcer syndrome.⁷ Ulcer healing and improvement was much better when omeprazole was used for squamous gastric ulcers than glandular gastric ulcers.^8,9

Warnings and Contraindications

Omeprazole’s safety during pregnancy has not been established, but a study in rats at doses of up to 34 times the recommended dose did not demonstrate any teratogenic effects; however, increased embryo–lethality has been noted in laboratory animals at very high doses.¹

Omeprazole is contraindicated in patients hypersensitive to it or other proton pump inhibitors. In patients with hepatic or renal disease, the drug’s elimination half-life may be prolonged, and dose adjustment may be necessary if the disease is severe.¹

There are numerous potential drug interactions for omeprazole. The following drug interactions have either been reported or are theoretical in humans or animals receiving omeprazole and may be of significance in veterinary patients. Unless otherwise noted, use together is not necessarily contraindicated, but the potential risks should be weighed and additional monitoring performed when appropriate.

BENZODIAZEPINES (eg, alprazolam, diazepam): Omeprazole may potentially alter benzodiazepine metabolism and prolong CNS effects.

CEFPODOXIME: Serum cefpodoxime concentrations may be reduced.

CEPHALEXIN: Omeprazole reduced peak cephalexin concentration and AUC, and delayed oral absorption in adult dogs; however, time > MIC was not altered.¹⁰

CLARITHROMYCIN: Increased levels of omeprazole, clarithromycin, and 14-hydroxyclarithromycin are possible.

CLOPIDOGREL: In healthy dogs, concurrent omeprazole increases formation of an inactive clopidogrel metabolite but without significant alteration of antiplatelet effect.¹¹ Pantoprazole should be considered in place of omeprazole if an interaction with clopidogrel is a concern.

CLORAZEPATE: Concurrent use with omeprazole may result in an increased risk for clorazepate toxicity.

CYANOCOBALAMIN (oral; Vitamin B12): Omeprazole may decrease oral absorption of cyanocobalamin.

CYCLOSPORINE: Omeprazole may reduce cyclosporine metabolism.

DIGOXIN: Omeprazole may increase digoxin bioavailability, increasing the risk for toxicity.

DIURETICS (eg, furosemide, hydrochlorothiazide): May increase risk of hypomagnesemia.

DRUGS REQUIRING DECREASED GASTRIC PH FOR OPTIMAL ABSORPTION (eg, ketoconazole, itraconazole, iron, ampicillin esters): Omeprazole may decrease drug absorption. If both drugs are used together, a pH-dependent medication should be administered 1 hour before omeprazole.

LEVOTHYROXINE: Concurrent use may decrease levothyroxine absorption.

METHOTREXATE: Concurrent use may increase methotrexate levels and risk of toxicity.

MYCOPHENOLATE: Mycophenolate peak concentration and total exposure (AUC) may be reduced.

PHENOBARBITAL: Concurrent use may increase risk for phenobarbital toxicity.

RIFAMPIN: Omeprazole serum levels may be decreased.

WARFARIN: Omeprazole may increase anticoagulant effect. Prothrombin time (PT) should be monitored in patients on concurrent warfarin therapy.
(Plumb’s Veterinary Drugs)

Misoprostol

Misoprostol is a prostaglandin E1 analog for treating or preventing gastric ulcers, especially associated with NSAIDs; may also be useful as an abortifacient adjunct. Misoprostol may be useful as primary or adjunctive therapy in treating or preventing GI adverse effects (anorexia, vomiting) or gastro-duodenal ulceration, especially when caused or aggravated by non-steroidal anti-inflammatory drugs (NSAIDs).¹

Misoprostol has two main pharmacologic effects that make it a potentially useful agent. By a direct action on parietal cells, it inhibits basal and nocturnal gastric acid secretion as well as gastric acid secretions that are stimulated by food, pentagastrin, or histamine. Pepsin secretion is decreased under basal conditions, but not when stimulated by histamine. Misoprostol also has a cytoprotective effect on gastric mucosa likely by increasing production of gastric mucosa and bicarbonate; increasing turnover and blood supply of gastric mucosal cells, misoprostol enhances mucosal defense mechanisms and healing in response to acid-related injuries.¹

There is some evidence of efficacy for misoprostol as an allergy medication in dogs, but due to the modest benefits, relatively high costs and adverse effects associated with these medications, they probably should not be used as first line medications to treat dogs with atopic dermatitis.¹²

For the treatment of gastric ulcers in horses, the recommended dosage of omeprazole is 4 mg/kg PO (paste form) every 24 hours for 4 weeks. To prevent recurrence, the recommended dosage is 2 mg/kg PO every 24 hours for at least another 4 weeks.¹ Omeprazole paste is typically dispensed in a plastic disposable syringe; to administer omeprazole paste for horses, set the syringe to the weight marking on the syringe plunger for the correct dose.

Where to buy Omeprazole+Misoprostol:

Omeprazole+Misoprostol is available in the U.S. through veterinary custom compounding companies.

Omeprazole 228 mg/ml / Misoprostol 0.14 mg/ml paste by NexGen is a superior solution for treating and preventing gastroduodenal ulcer disease in horses.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Plumb’s Veterinary Drugs.

²EquiNews.

³Bush J, van den Boom R, Franklin S. Comparison of aloe vera and omeprazole in the treatment of equine gastric ulcer syndrome. Equine Veterinary Journal. 2018;50(1):34-40.

⁴Kerbyson NC, Knottenbelt DK, Carslake HB, Conwell RC, Sutton DGM, Parkin TDH. A Comparison Between Omeprazole and a Dietary Supplement for the Management of Squamous Gastric Ulceration in Horses. Journal of Equine Veterinary Science. 2016;40:94-101.

⁵Mason LV, Moroney JR, Mason RJ. Prophylactic therapy with omeprazole for prevention of equine gastric ulcer syndrome (EGUS) in horses in active training: A meta-analysis. Equine Veterinary Journal. 2019;51(1):11-19.

⁶Endo Y, Tsuchiya T, Sato F, et al. Efficacy of omeprazole paste in the prevention of gastric ulcers in 2 years old thoroughbreds. J Vet Med Sci. 2012;74(8):1079-1081.

⁷Sykes BW, Hewetson M, Hepburn RJ, Luthersson N, Tamzali Y. European College of Equine Internal Medicine Consensus Statement-Equine Gastric Ulcer Syndrome in Adult Horses. J Vet Intern Med. 2015;29(5):1288-1299.

⁸Sykes BW, Sykes KM, Hallowell GD. A comparison of three doses of omeprazole in the treatment of gastric ulceration in throughbred racehorses. J Vet Intern Med. 2013;27(3):652-652.

⁹Sykes BW, Sykes K, Hallowell GD. Comparison of the effect of two doses of omeprazole on the squamous gastric mucosa in thoroughbred racehorses. Vet Rec. 2014;175(10).

¹⁰Lupi MP, Passini S, Montoya L, Albarellos G. Influence of the administration of omeprazole on the oral absorption of cephalexin: differences between adults and aged dogs. J Vet Pharmacol Ther. 2015;38:100-100.

¹¹Thames BE, Lovvorn J, Papich MG, et al. The effects of clopidogrel and omeprazole on platelet function in normal dogs. J Vet Pharmacol Ther. 2017;40(2):130-139.

¹²Olivry T, et al. Treatment of canine atopic dermatitis: 2010 clinical practice guidelines from the International Task Force on Canine Atopic Dermatitis. Veterinary Dermatology. 2010;21(3):233-248.