Horses occupy a distinct niche in veterinary pharmacovigilance. They are large, long-lived, athletic animals treated with a drug formulary that spans companion-animal products (NSAIDs, sedatives) and livestock products (antiparasitics, antibiotics). Their adverse-event profile reflects that duality.

This article analyzes 18,126 adverse-event reports for which the reported species is Horse, drawn from the FDA Center for Veterinary Medicine (CVM) animal drug adverse-event database (1987 through early 2026). Every number below comes from a direct computation of the FDA CVM adverse-event reports (analysis run date: 2026-06-09).

How equine pharmacovigilance differs

Three factors make horse adverse events distinct from dog and cat reports:

1. Antiparasitic reactions dominate. Ivermectin — alone and combined with praziquantel — is the top-reported drug class. The characteristic equine reaction is lip and tongue edema following oral paste administration, not seen in other species.
2. Efficacy complaints are prominent. Lack-of-efficacy reports for anti-ulcer drugs (omeprazole), antiparasitics, and sedatives collectively make up a large share of equine reports, reflecting the performance demands of sport and working horses.
3. The fatality rate is high. Combined death and euthanasia outcomes represent 13.7% of equine reports, driven by colic, severe neurotoxicity (moxidectin), and infectious-disease treatment failures.

Top 20 active ingredients

The list clusters into four therapeutic classes: antiparasitics (ivermectin, moxidectin, pyrantel, praziquantel), GI therapeutics (omeprazole, sucralfate), sedatives and anesthetics (detomidine, ketamine, xylazine), and anti-inflammatory/joint therapies (flunixin, firocoxib, hyaluronate, PSGAG, clodronate).

Pergolide mesylate at rank 5 reflects the prevalence of equine pituitary pars intermedia dysfunction (PPID, or Cushing's disease) in the aging horse population and the daily, lifelong medication requirement.

Top reactions

Death is the single most-reported reaction in horses (8.2%) — a striking difference from dogs and cats, where vomiting leads. Abdominal pain and colic together (6.7% + 4.0% = 10.7%) reflect the GI vulnerability of the equine digestive system. Lip edema (4.7%) is almost exclusively an ivermectin-paste reaction in horses.

Outcomes

The combined Died + Euthanized rate is 13.6% (2,468 unique reports, after deduplicating entries listing both outcomes). This is higher than the overall database average and reflects both the severity of equine medical emergencies (colic, catastrophic musculoskeletal injury) and the economic and logistical realities of treating critically ill horses.

Ingredient deep dives

Ivermectin — the lip-edema signal

Ivermectin (including the ivermectin/praziquantel combination) is the most-reported drug class in horses. Combined across all ivermectin-containing entries, it accounts for approximately 3,377 reports — 18.6% of all equine reports.

The combination product (ivermectin + praziquantel) shows a distinctive reaction pattern dominated by oral cavity reactions:

• Lip edema: 47.8%
• Application site swelling: 36.2%
• Hypersalivation: 14.6%
• Tongue edema: 13.1%

This reaction profile is a well-recognized response to the oral paste formulation — the vehicle or the active ingredients cause local inflammation of the lips and oral mucosa. While alarming, most cases resolve without lasting harm. The fatality rate for this ingredient is low (3.7% Died).

Omeprazole — the efficacy question

Omeprazole (all formulations) generates approximately 1,738 reports combined. The dominant reaction is therapeutic failure:

• Ineffective — gastric ulcer(s): 45.6%
• Lack of efficacy — NOS: 40.8%
• Partial lack of efficacy: 11.8%

This means over 85% of omeprazole reports are efficacy complaints rather than safety events. The high rate reflects several factors: gastric ulceration in horses is common and recurrent, owners and trainers have high expectations for symptom resolution, and endoscopic follow-up may reveal persistent ulcers despite treatment. The death rate is very low (0.6% Died).

The "omeprazole 37% oral paste" at rank 20 (218 reports) is a higher-strength formulation — likely reflecting the concentrated use in performance horses and the dosing precision challenges of paste products.

Moxidectin — the neurotoxicity signal

Moxidectin's 1,368 reports reveal the most concerning safety profile among the top equine antiparasitics:

• Ataxia: 17.4%
• Locking mechanism abnormal (stifle): 16.7%
• Depression: 16.3%
• Recumbency: 9.4%
• Died: 11.4% (combined with 1.6% Euthanized = 13.0%)

Moxidectin is a macrocyclic lactone with a narrower safety margin than ivermectin, particularly in young, underweight, or heavily parasitized horses. The neurological signs (ataxia, depression, recumbency) reflect its mechanism of action on GABA-gated chloride channels. The high fatality rate relative to ivermectin products is a known pharmacovigilance signal that appears consistently in the data.

Pergolide mesylate — managing PPID

Pergolide (1,152 reports) is the drug most closely associated with a single disease: equine pituitary pars intermedia dysfunction. Its reaction profile is the adverse-event footprint of managing a chronic endocrine disease in an aging horse population:

• Anorexia: 19.2%
• Diarrhea: 9.5%
• Lethargy: 9.0%
• Behavioral disorder: 7.1%
• Lack of efficacy: 6.7%
• Weight loss: 6.0%

The anorexia rate is notable — pergolide-induced inappetence is one of the most common reasons for dose adjustment in PPID management. The 6.7% lack-of-efficacy reports reflect the challenge of stabilizing advanced PPID cases and the variability in dopaminergic response.

Breed distribution

Quarter Horses dominate the breed distribution at 23.0%, reflecting their status as the most numerous horse breed in the United States. Thoroughbreds (11.3%) are overrepresented relative to their population share, likely due to the intense pharmacological management of racing and sport horses.

Year-over-year trend

Equine adverse-event reporting has been relatively stable at 550–820 reports per year since 2000, with a gradual increase in recent years:

• 2015: 748
• 2020: 673
• 2024: 819
• 2025: 801

The consistency is notable — unlike companion-animal reports, which have grown substantially with population growth, equine reports reflect a more stable (and in the US, declining) horse population. The slight recent increase may reflect greater awareness of FDA reporting pathways among equine practitioners.

What the equine data means for practice

Antiparasitic reactions are formulation-specific. The lip-edema reaction to ivermectin paste and the neurotoxicity of moxidectin are distinct pharmacovigilance signals unique to equine practice. Dosing accuracy with paste formulations is a known risk factor.

Efficacy reporting is disproportionately equine. The omeprazole data shows that equine adverse-event reporting captures therapeutic failure at rates far beyond what is seen in small-animal reports. This reflects the performance expectations of equine sports medicine.

The fatality rate is real. At 13.7% combined Died + Euthanized, equine reports carry more severe outcomes than the overall database. Colic, neurotoxicity, and the economic limitations of intensive-care treatment for horses all contribute.

Sources

• FDA Center for Veterinary Medicine, Adverse Event Reporting System. openFDA Animal Drug Adverse Event API. Available at: https://open.fda.gov/apis/animalandveterinary/event/
• FDA CVM, "Adverse Event Reports for Animal Drugs and Devices." Available at: https://www.fda.gov/animal-veterinary/product-safety-information/adverse-event-reports-animal-drugs-and-devices
• FDA CVM, "Veterinary Adverse Event Reporting for Manufacturers." Available at: https://www.fda.gov/animal-veterinary/report-problem/veterinary-adverse-event-reporting-manufacturers
• Merck Veterinary Manual, "Antiparasitic Drugs — Macrocyclic Lactones." Available at: https://www.merckvetmanual.com/pharmacology/anthelmintics/anthelmintics
• FDA CVM, "How to Report Animal Drug and Device Side Effects and Product Problems." Available at: https://www.fda.gov/animal-veterinary/report-problem/how-report-animal-drug-and-device-side-effects-and-product-problems