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Omeprazole (Prilosec, GastroGard)
Revised: August 24, 2022
Published: June 26, 2006

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Prilosec, GastroGard, Zegerid, Ulcergard

Available in oral paste, oral suspension (liquid), or sustained-release capsules (10 mg, 20 mg, 40 mg)


Stomach ulceration in humans is a prominent medical condition, and there has long been pressure to develop effective and convenient ways to control it. Until relatively recently, we relied on simply neutralizing stomach acid by pouring alkaline solutions (i.e., Alka Seltzer, Tums, Rolaids, etc.) into the stomach. In fact, ulceration is a complicated process and not so easily solved. 

A more modern approach involves reducing the secretion of acid in the stomach rather than simply trying to neutralize it. The H2 blocker class of antacids (cimetidine, famotidine, and others) was developed to reduce the effects of histamine on stomach acid secretion. This represented a breakthrough, and many people and animals have helped the stomach acid block was still incomplete. The quest continued for a more complete block in acid secretion. 

Omeprazole represents a different tact: proton pump inhibition. The quantity of acid ultimately amounts to the number of protons. The proton pump is central to secreting acid into the stomach, and with this pump inhibited, stomach acid production is halted. Proton pump inhibitors like omeprazole are the strongest antacids of all. They work not only when the stomach is full/digesting food but all the time.

How this Medication is Used

Omeprazole is used in the treatment of stomach ulcers or in the prevention of stomach ulcers. Relatedly, omeprazole is commonly used as part of "triple therapy" against the ulcer-causing stomach bacteria known as Helicobacter. Omeprazole is generally used once a day though it can take 3 to 5 days to achieve maximum effect. Omeprazole is best given on an empty stomach before the first meal of the day but may be given with food if necessary.

It is important to remember, that stomach acid has important functions in the digestion of food and in protection against bacterial colonization. We do not want to obliterate the stomach's ability to produce acid long-term. For this reason, omeprazole is best used for periods of four weeks or less.

If a dose is accidentally skipped, simply give it when it is remembered and time the next dose accordingly.

Side Effects

Increases in liver enzymes may be induced by the use of omeprazole. This is not harmful but should be recognized as an omeprazole reaction should it be seen on a blood test.

Side effects of omeprazole include appetite loss, gas, and diarrhea, but these are generally mild. 

Omeprazole can reduce cerebrospinal fluid production though it is not known how. While this is listed as a side effect, it could be the desired primary effect if used in patients with hydrocephalus or syringomyelia where the goal is to reduce cerebrospinal fluid pressure.

Ironically, nausea, vomiting, diarrhea, and flatulence are occasionally reported as side effects.

Interactions with other Drugs

Omeprazole inhibits a system of liver enzymes called the cytochrome p450 system. Many other drugs depend on this system for removal. This means that if omeprazole is used, the following drugs will last longer and work stronger: benzodiazepine tranquilizers (diazepam, alprazolam, etc.), phenytoin (used to treat both seizures and heart rhythm disturbances), and warfarin (a blood thinning agent).

Because omeprazole reduces stomach acid, other drugs that require an acid environment for absorption into the body may not be as well absorbed. Such drugs include ketoconazole (an antifungal agent), and some forms of the antibiotic ampicillin.

Blood levels of cyclosporine, an immunomodulator, may be increased with the concurrent use of omeprazole.

Omeprazole may inhibit the activation of clopridogrel, an anti-coagulant medication.

Clopidogrel, a blood thinner, may not be activated properly if used with omeprazole at the same time. It may be best to use a different antacid.

Concerns and Cautions

Omeprazole should be stored at room temperature in a light-tight container.

If your pet is reluctant to swallow the capsules, do not attempt to crush the pellets inside the capsules and add them to water to food. It is permissible to add them to a small amount of fruit juice, however.

Omeprazole is removed from the body by both the liver and kidneys. The dosage of omeprazole should be altered if the patient suffers from either liver or kidney insufficiency.

The safety of the use of this medication in lactation or pregnancy has not been established.

Omeprazole requires three to five days to achieve maximum effect in dogs. Information on cat timing is lacking at this time.

Four weeks of use is considered safe for cats and dogs but such extreme stomach acid reduction for longer than this period is controversial.

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