Last Updated: April 3, 2025
Gastric ulcers are a significant concern in the world of horse health, with prevalence rates ranging from 50% at the low end to 90% at the high end. Even at the lowest estimates, it’s a pretty rough statistic to wrap your head around the idea that at least half of all horses, of all ages, have stomach ulcers.
Today, many horse owners are aware that Equine Gastric Ulcer Syndrome is preventable with management strategies targeting feed, such as consistent access to forage, lifestyle, including social interaction and some form of turnout, and reducing stress levels. However, despite our best efforts, sometimes gastric ulcers rear their ugly head no matter what, especially in performance horses.
Like any athlete, extra care is often required, and for horses, this typically calls for veterinary intervention. The most common medication for treating ulcers in horses is omeprazole, and GastroGard® is the only FDA-approved omeprazole treatment for gastric ulcers in the U.S., and requires a veterinarian’s prescription. (UlcerGard® is a lower-dose omeprazole that has FDA approval for preventing gastric ulcers, and is available over the counter). To treat stomach ulcers, many veterinarians will prescribe a 4-week GastroGard protocol, based on the label use directions, and this may continue for an additional 4 weeks to prevent recurrence. Working with a veterinarian for diagnosis and proper treatment of gastric ulcers is critical.
While omeprazole has been vital in the treatment and prevention of equine gastric ulcers for nearly three decades, veterinarians and scientists now also recognize that some horses may experience something called the ‘acid rebound period’ when treatment with omeprazole is discontinued. Below we share more about this well-known phenomenon, including some of the research around it, and what horse owners need to know to support their horses’ gastric health when omeprazole treatment ends.
How Does Omeprazole Work?
Omeprazole falls in a class of drugs known as proton pump inhibitors (PPI’s), which means that it works by binding to the acid-producing pumps in the stomach to block the final step of hydrochloric acid (HCl) production. Through this mechanism, omeprazole effectively reduces stomach acid levels, which in turn gives the horse’s stomach a chance to heal. It is not an antacid nor does it provide a physical buffer against stomach acid, but rather it is absorbed systemically to turn off the proton pumps.
What is the ‘Acid Rebound Period?’
While omeprazole is extremely effective at treating gastric ulcers because it essentially removes stomach acid from the equation so that gastric tissues can heal, some horses may experience downstream challenges as the result of omeprazole treatment (especially after extended treatment duration).
The primary concern is known as rebound acid hypersecretion (RAHS), or the ‘acid rebound period’. This describes a phenomenon in which gastric acid production increases following the end of omeprazole or other acid-suppressive treatments. This may expose vulnerable, newly-healed gastric tissue to a “surge” in acid, and in some cases leads to rapid re-occurrence of the ulcers that have just healed. In addition to the obvious health concerns and discomfort this presents to the horse, this situation is also frustrating to horse owners and veterinarians given the time, management, and financial commitment that goes into omeprazole treatment to begin with.
First, we have to understand the anatomy of the equine stomach to fully grasp the concept of RAHS.
The horse’s stomach is divided into two distinct regions that are very much Jekyll to Hyde:
1) The Squamous, or Non-Glandular, Region is located in the upper part of the stomach, and is lined with stratified squamous epithelium. In other words, it is composed of a type of tissue that is similar to the esophagus. As its name implies, the squamous, or non-glandular, region does not have secretory glands. Secretory glands can secrete substances to protect tissue from gastric acid, so the lack of these glands makes this area of the stomach very susceptible to ulceration.
2) The Glandular Region, in contrast, is located in the lower portion of the stomach, and is exactly opposite. This region contains gastric glands that continuously secrete hydrochloric acid (HCl) and digestive enzymes like pepsin, which is essential for initiating protein digestion. The glandular region also produces mucus and bicarbonate, which protect the stomach lining from the harsh acidic environment. In other words, this region is more protected from gastric ulceration, although it still carries some risk.
Gastric acid, or HCl, is necessary for protein digestion, among other roles. It gets a bad rap, but it truly is essential for nutrient digestion. In humans, HCl is produced in anticipation of a meal as needed, but in horses, who are designed to be 24/7/365 grazers, HCl production is always happening! It’s like a light switch that can only be in the “on” position.
Gastrin, a hormone produced by cells in the horse’s stomach, stimulates acid secretion. In horses, gastrin levels fluctuate based on the stomach's contents, but they never drop to zero; this ensures a baseline level of acid production at all times.
When horses receive omeprazole, the medication suppresses acid production. In turn, gastrin production increases to compensate for the suppressed acid levels. All of this means that when the omeprazole treatment ends, the increased gastrin levels can cause a surge in acid production, or what is known as RAHS.
In fact, research reports that ulcers can return to pre-treatment levels in as little as 3 days after ending a course of omeprazole. Interestingly, this same research study found that administration of a supplement containing probiotics and prebiotics on the two days after ending omeprazole, assisted in reducing the rate of ulcer recurrence.
Another recent study found that gastrin levels return to normal within 2-4 days after ending omeprazole treatment, which further supports the idea that there is a critical time period in which horses ending omeprazole treatment may benefit from additional protection.
How to Mitigate RAHS in Horses
Tapering off of omeprazole has not proven to be especially helpful for reducing the risk of RAHS, and it creates higher costs for horse owners. Therefore, this is not a great strategy.
Current recommendations for reducing RAHS focus on implementing management and feed changes during omeprazole administration, to set your horse up for success post-omeprazole. This list is very similar to the list of strategies for reducing the risk of gastric ulcers in the first place (wink wink):
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Feed more frequent, smaller meals throughout the day.
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If possible, provide constant access to forage. If your horse is overweight or at risk for metabolic conditions, work with an equine nutritionist and veterinarian who can help you test hay to find a suitable high fiber, low nutrient option.
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Feed grain in smaller amounts to reduce the amount of sugar fed at one time.
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If your horse requires more calories, consider feeding healthy fats.
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Feed your horse small amounts of alfalfa hay prior to exercise. Alfalfa contains high levels of calcium and protein, which help buffer and protect the stomach from “acid splash” during exercise.
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Provide your horse with social interaction that they enjoy. Everyone needs good friends for optimal health!
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If possible, increase turnout time (assuming that this is a non-stressful event for your horse).
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Work to understand your horse’s specific needs. Easier said than done, but if there is an enrichment toy that your horse loves, or a way to gradually acclimate them to a change, do your best to accommodate.
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Offer research-supported gut health supplements that can help balance the overall GI environment and act as one layer of protection, particularly in the days directly following the end of omeprazole treatment.
In Summary
Helping the horse with gastric ulcers is not ‘cut and dry,’ and because horses’ stomachs are always producing acid, managing gastric health requires an approach that aligns with their natural physiology.
While omeprazole prescribed by a veterinarian remains a cornerstone in the treatment of Equine Gastric Ulcer Syndrome, proactive management is essential for preventing rebound acid hypersecretion and ulcer recurrence. Remember, you know your horse best. Work with your veterinarian and equine nutritionist to find solutions that suit your horse. You are their advocate and their voice.
Research Citations
Shan R, Steel CM, Sykes B. The Impact of Two Recommended Withholding Periods for Omeprazole and the Use of a Nutraceutical Supplement on Recurrence of Equine Gastric Ulcer Syndrome in Thoroughbred Racehorses. Animals. 2023; 13(11):1823. https://doi.org/10.3390/ani13111823
Clark, B., Steel, C., Vokes, J., Shan, J. R., Gedye, K., Lovett, A., & Sykes, B. W. Evaluation of the effects of medium-term (57-day) omeprazole administration and of omeprazole discontinuation on serum gastrin and serum chromogranin A concentrations in the horse. Journal of Veterinary Internal Medicine. 2023; 37(4): 1537–1543. https://doi.org/10.1111/jvim.16795
