Omeprazole is an FDA-approved drug that is sold under the tradenames GastroGard and UlcerGard. Omeprazole is used to prevent or treat equine gastric ulcer syndrome (EGUS).
Gastric ulceration is a painful condition with an extremely high prevalence in horses. Up to 90% of performance horses are affected by EGUS. High rates of pleasure horses are affected as well.
Ulcers are sores or lesions that develop in the intestinal lining of the horse. They can cause your horse to become girthy, resistant to training, agitated and generally crabby.
Omeprazole treats ulcers by suppressing the production of stomach acid. It is a proton pump inhibitor medication, meaning that it temporarily reduces the acidity of the stomach.
Treatment for gastric ulcers can be a long and expensive commitment. Unfortunately, following treatment, there is a risk of ulcer recurrence unless changes in your horse’s management and feeding program are made.
Horse owners should be aware of how omeprazole works to prevent and heal ulcers, as well as any complications that could occur with treatment, such as rebound acid hypersecretion (RAH).
How to Treat Ulcers in Horses
Why do horses get ulcers and how can you prevent or treat them?
The cause of EGUS is often multi-factorial, meaning several interacting risk factors can cause ulcers to develop. These risk factors can include diet, exposure to stress, workload, and environment.
The stomach of the horse is a highly acidic environment. Proton pumps in the stomach continuously produce acids such as hydrochloric acid (HCl) to aid in the breakdown of food.
This process occurs whether or not there is food in the stomach to digest. Over a single day, a typical 500 kg (1100 lbs) horse can produce up to 60 litres (16 gallons) of gastric acids.
The equine stomach is divided into two sections: the upper squamous region and the glandular region.
The glandular region produces mucous and bicarbonate which protects the stomach lining by naturally buffering acids.
The upper squamous region is not so lucky. This region does not produce mucous. Instead, it relies on food and saliva to buffer acids.
If a horse is grazing throughout the day, then the upper squamous region is naturally protected by food and saliva. However, if the horseâ€™s stomach is empty, this region becomes vulnerable to the acidic environment.
This can result in painful lesions and sores develop along the wall of the digestive tract.
When ulcerations develop in the stomach, it is referred to as EGUS. When ulcers occur in the hindgut, it is referred to as right dorsal colitis.
Common Causes of Equine Ulcers
There are many potential contributing factors for ulcers including:
- Diet composition
- Feeding regimen
- High intensity exercise
- Environmental changes
- Social stress
- Anti-inflammatory drug use (ie NSAIDs)
Knowing some of the causes of EGUS can help horse owners make changes to naturally prevent ulcers from occurring.
Signs & Symptoms
Not all cases of gastric ulceration are symptomatic. This means your horse may have ulcers without displaying any outward signs.
However, most horses will show signs of ulcers including changes in behaviour, reluctance to work, girthiness or irritability.
If you begin to notice some of these potential signs and symptoms of ulcers, then a visit from your vet is warranted.
Signs and symptoms to watch for include:
- Reduced appetite
- Chronic diarrhea
- Weight loss or poor body condition
- Rough coat or poor coat quality
- Reduced performance
- Aggressive or nervous disposition
- Cribbing and other stereotypic behaviours
To determine if your horse has ulcers, your veterinarian will perform a gastroscopy. This procedure uses a small, flexible tube with a camera at the end to give your vet an inside look at the esophagus, stomach, and upper intestine.
Your vet will be able to determine if ulcers are present as well as their severity. Gastroscopy can visually assess the number and severity of ulcers which can help decide the best treatment options for your horse.
The only definitive way to diagnose EGUS is via gastroscopic examination by a veterinarian. However, this procedure can be expensive and may not be accessible to all horse owners.
Given the extremely high prevalence of ulcers, many horse owners will assume that their horse is affected by ulcers if they display the above telltale signs.
Even if your horse has not been diagnosed with ulcers, it is advised to implement measures to prevent ulcers as part of your horseâ€™s everyday routine.
How Does Omeprazole Work for Ulcers?
The most common treatment for ulcers is vet-prescribed omeprazole. There are two products available that contain omeprazole for equine ulcers: GatsroGard and UlcerGard.
GastroGard is the FDA-approved treatment for EGUS and is restricted to use with a prescription issued by a licensed veterinarian. It comes in the form of an oral paste.
UlcerGard is an over-the-counter version that horse owners can use at smaller doses to prevent EGUS.
However, recent research suggests that omeprazole may not be effective at treating ulcers in the glandular region of the stomach.  There are also adverse effects to consider.
If your horse is currently being treated with GastroGard or UlcerGard, it is important to consider dietary and management changes to help reduce ulcer risk and support digestive health.
Mechanism of Action
Omeprazole is a proton pump inhibitor. It blocks the production of gastric acid, increasing the pH level in the stomach.
Omeprazole passes through the stomach to the small intestine where it is absorbed into the bloodstream. It then gets taken up by acid-producing cells of the stomach, called parietal cells, and interferes with their function.
In the stomach, many proton pumps trade hydrogen ions for potassium ions using the hydrogen-potassium ATPase enzyme.
Omeprazole binds to this enzyme and prevents the pump from working properly. This inhibits the secretion of gastric acid, helping to make the stomach environment less acidic.
When Omeprazole binds to acid-secreting cells, it irreversibly stops them from producing stomach acid. However, these cells are constantly turned over meaning that old cells die off and new cells are formed.
This is why Omeprazole must be administered continuously in order to have an effect. On-going treatment is necessary to control gastric acid secretion.
By reducing acidity, Omeprazole can support tissue repair and give the stomach mucous lining time to heal.
Dosage and Administration
GastroGard Paste is administered orally once a day at a dose of 4 mg/kg body weight.  For a 500 kg horse, this would equate to 2000 mg of GastroGard.
The concentration of omeprazole in GastroGard is 2.28 g per 6.15 g tube, or 37%. This means that the 500 kg horse receiving 2000 mg GastroGard is consuming 740 mg omeprazole.
Treatment typically lasts up to four weeks or 28 days. It may be recommended to continue treatment for another four weeks at a lower dose (2 mg/kg body weight).
This extended treatment plan is designed to wean your horse off of omeprazole slowly to reduce the risk of acid rebound. Acid rebound following cessation of Omeprazole is a major factor in the recurrence of ulcers.
UlcerGard is used to prevent ulcers in horses that are at high risk of this condition. It is administered at a much lower dose.
The recommended dose of UlcerGard is 1 mg/kg body weight (or 500 mg for a 500 kg horse).
UlcerGard contains 2.28 g omeprazole per 6.15 g tube (37% omeprazole). Therefore, a 500 kg horse is receiving 185 mg omeprazole per serving.
Directions for Use
Both GastroGard and UlcerGard are administered orally via syringe to your horse. 
Adjust the syringe to the correct weight of the horse to administer treatment at 4 mg/kg body weight. For 2 mg/kg bodyweight administration, adjust according to 50%, or half, of the horseâ€™s weight.
There should be no feed in the horse’s mouth when administering omeprazole. Insert the syringe into the interdental space in the horseâ€™s mouth.
This will ensure the active ingredient reaches the back of the tongue or the cheek pouch.
Make sure the horse consumes the full dose and that none is lost or rejected.
Replace the cap and store it for reuse if the tube is not empty after dosing. Store at 20-25 degrees Celsius (68-77 degrees Fahrenheit).
A tube (6.15 g, or 6150 mg) of GastroGard costs approximately $35.00 USD or $50.00 CAD.
One tube will last a 500 kg horse for three days. A typical four-week treatment program will require ten tubes with a minimum cost of $350 USD or $500 CAD.
This does not include the cost of veterinary diagnosis, check-ups, and an additional month of GastroGard to wean your horse and prevent rebound acid hypersecretion.
It can easily cost horse owners over $1000 to treat an EGUS diagnosis. This makes prevention of the condition even more important.
Omeprazole Adverse Effects
Omeprazole is effective for treating EGUS, but is associated with a risk of side effects, particularly if used for long periods of time.
Researchers have previously documented the following adverse effects associated with omeprazole administration.
1. Rebound Acid Hypersecretion
The major concern with using proton pump inhibitors such as omeprazole is rebound acid hypersecretion, or acid rebound. 
Acid rebound occurs when treatment with omeprazole ends abruptly. Stomach cells begin to hyper-secrete gastric acids, resulting in a lower gastric pH.
This phenomenon is attributed to high levels of gastrin, a regulatory hormone. Gastrin is released to stimulate gastric acid secretion. 
When omeprazole is administered, proton pumps are blocked from secreting gastric acids. However, this does not stop the production of gastrin.
After treatment, the proton pumps begin to function again and can respond by over-producing hydrochloric acid and other stomach acids.
Additionally, there is a large build-up of gastrin in the body. Researchers report that gastrin levels in horses doubled after only 14 days of omeprazole administration. 
This exacerbates the production of gastric acids creating an unnaturally low pH (excessive acidity) in the stomach.
The mucous lining is then exposed to this hyper-acidic environment, and new lesions can form, often resulting in another EGUS diagnosis.
This can be especially frustrating for horse owners after paying the high cost of treating ulcers with Omeprazole only to see their horses require another round of treatment.
2. Reduced Protein Digestibility
The stomach is acidic for a reason. Acids help break down foods into smaller particles to allow digestive enzymes to release nutrients for absorption.
The stomach is the first phase of chemical digestion, which is the process by which foods are broken down into nutrients that can be absorbed and utilized by the body.
Stomach acids activate many digestive enzymes, such as pepsin. Pepsin is an enzyme that breaks down proteins into smaller peptides before they enter the small intestine.
Pepsin requires stomach acid to be activated. If you suppress stomach acid secretion by administering omeprazole, you can unintentionally reduce the activation of pepsin and negatively impact the digestibility of proteins.
Arabian horses treated with omeprazole for 11 days exhibited poor protein metabolism compared to a control group. 
In the short-term, this may not have a significant impact on your horse’s wellbeing. However, long-term use of Omeprazole may result in secondary protein deficiencies which could impact muscle development, growth, performance and wound healing.
3. Reduced Mineral Digestibility
Calcium and magnesium absorption in horses is suppressed by omeprazole usage. Digestive enzymes involved in the processing of calcium and magnesium also rely on stomach acids for activation.
Researchers found that omeprazole suppressed the absorption of magnesium in the small intestine in cell culture models. 
In a research trial involving horses, calcium digestibility was reduced after 21 days of omeprazole administration (4 mg/kg body weight). This study did not note a difference in the digestibility of magnesium or other minerals. 
Calcium and magnesium are important minerals in horses for bone structure and integrity. They also play key roles in muscle maintenance and function. Additionally, calcium and magnesium help buffer stomach acids.
Impaired calcium and magnesium absorption could negatively impact your horse’s health if it creates a deficiency in these minerals.
In humans, long-term use of proton pump inhibitors, such as omeprazole, appears to increase the risk of bone fractures.  This has not yet been confirmed in horses.
Long-term studies in horses treated with omeprazole are warranted to understand potential side effects on nutrient digestibility and mineral absorption.
4. Increase in Pathogenic Bacteria
The acidic environment of the stomach helps to protect your horse from pathogenic bacteria. Many pathogens cannot survive the harsh pH of the stomach, helping to support your horse’s immune function.
Omeprazole works by raising the gut pH. The higher pH levels could increase the risk of pathogenic bacteria colonizing the stomach and penetrating the mucosal barrier which can damage the tissue and activate immune responses.
In humans, omeprazole administration appears to increase gastric bacterial growth.  This phenomenon is hypothesized to affect horses as well, but studies have found mixed results.
In one study, after only seven days of omeprazole administration in horses, changes in the types of microbes in the stomach were found. However, this study did not find increased overall numbers of microbes in these horses. 
In another study, horses treated with the recommended dose of omeprazole (4 mg/kg) for 28 days did not have significantly different gastric microbial growth. The researchers reported minor differences in composition and diversity. However, this was not clinically significant. 
Further study is needed to understand how omeprazole use in horses affects the gastric microbiota.
5. Potential Complications
Omeprazole side effects are not listed in the manufacturer’s information on this drug for horses. However, it has been associated with certain adverse effects in humans.
Some of these reported side effects of omeprazole use in humans include:
- Stomach pain
- Vitamin B-12 deficiency
- Kidney damage
- Polyps or growths on stomach lining
Preventing Gastric Ulcers
As Benjamin Franklin said, “An ounce of prevention is worth a pound of cure”.
Given the high cost of treating gastric ulcers with omeprazole and the risk of rebound and other side effects, it’s important to consider ways to naturally reduce your horse’s risk of ulcers.
Consider making some changes to your horse’s feeding program, environment and routine. There are many safe and natural strategies to prevent ulcers from developing your horse, including:
- Ensure constant access to water
- Avoid long periods with an empty stomach
- Always exercise your horse after feeding
- Feed a fibre-rich diet and high quality hay
- Provide high quality protein
- Minimize long-term NSAID use
- Limit travel when possible
- Increase turnout and grazing access
- Give your horse socialization
- Provide supplements for digestive health
Visceral+ for Acid Rebound
There are a number of proven natural ingredients that can support your horse’s gut health and reduce the risk of ulcer recurrence.
Mad Barnâ€™s Visceral+ supplement was developed with safe, natural ingredients to help horses maintain healthy stomach tissue.
Visceral+ was formulated in consultation with veterinarians who were frustrated with the high rate of post-treatment relapse in horses using GastroGard for ulcers.
Visceral+ is clinically studied in horses with ulcers. It is designed to nourish the equine microbiome and support stomach barrier function without interfering with the natural production of stomach acid.
Ingredients such as Saccharomyces cerevisiae yeast, lecithin, magnesium, milk thistle extract, meadowsweet, slippery elm, and probiotics work to:
- Support the natural barrier that protects stomach tissue from gastric acids
- Supporting digestive comfort to maintain a calm horse
- Nourish your horseâ€™s microbiome and support the healing of stomach tissue
- Promote defenses against pathogens by helping to maintain immune health
Before altering your feeding program, you should consult with a qualified nutritionist. Submit your horseâ€™s diet online, and one of our equine nutritionists will provide a complimentary review.
If you believe your horse may be affected by ulcers, contact your veterinarian for a full diagnosis. Your veterinarian can help you better understand available treatment options, including omeprazole, and how to minimize the risk of ulcer rebound.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Wise J.C. et al. Pharmacokinetic and pharmacodynamic effects of 2 registered omeprazole preparations and varying dose rates in horses. J Vet Intern Med. 2020.
- Andrews F.M. et al. Efficacy of omeprazole paste in the treatment and prevention of gastric ulcers in horses. Equine Vet. 1999.
- Sykes B.W. A free ride: is long-term omeprazole therapy safe and effective. Br Equine Vet Edu. 2021.
- Merial Inc. GastroGard-omeprazole paste. 2018.
- Pagan J.D. et al. Omeprazole reduces calcium digestibility in Thoroughbred horses. J Equine Vet Sci. 2020.
- Melo S.K.M. et al. A proton-pump inhibitor modifies the concentration of digestion biomarkers in healthy horses. J Equine Vet Sci. 2014.
- Thongon N. & Krishnamra N. Omeprazole decreases magnesium transport across Caco-2 monolayers. World J Gastroenterol. 2011.
- Hansen K.E. et al. Do proton pump inhibitors decrease calcium absorption? J Bone Mineral Research. 2010.
- Sharma B.K. et al. Intragastric bacterial activity and nitrosation before, during, and after treatment with omeprazole. Br Med J. 1984.
- Tyma J.F. et al. Investigation of effects of omeprazole on the fecal and gastric microbiota of healthy adult horses. Am J Vet Res. 2019.
- Cerri S. et al. Effect of oral administration of omeprazole on the microbiota od the gastric glandular mucosa and feces of healthy horses. J Vet Intern Med. 2020