Key Insights
  • Acepromazine is a veterinary sedative used to reduce anxiety and make horses safer to handle
  • Acepromazine reduces dopamine activity in the nervous system, which decreases movement and reactivity
  • Injectable acepromazine is approved for horses, while oral tablets may be prescribed off-label by veterinarians
  • Male horses should be monitored for penile prolapse or delayed penile retraction after acepromazine administration
  • Acepromazine is prohibited during competition under FEI rules and may be restricted by other organizations

Acepromazine is one of the most commonly used sedatives in equine veterinary medicine. It is valued for its ability to reduce anxiety and make horses easier and safer to handle during veterinary procedures, farrier care, transportation, and other potentially stressful situations.

Acepromazine primarily affects dopamine receptors in the brain, producing a calming effect without the profound sedation or pain-relieving effects associated with some other sedative and anesthetic drugs.

Although it is generally considered safe when prescribed by a veterinarian, acepromazine is not appropriate for every horse. Age, hydration status, cardiovascular health, temperament, and intended use all influence whether this medication is the right choice.

Keep reading to learn how acepromazine works in horses, common veterinary uses, administration methods, potential side effects, safety considerations, and what owners should know about competition rules.

This article is intended for educational purposes only. The information presented here is not a substitute for veterinary advice.

Acepromazine for Horses

Acepromazine is an approved sedative and anxiolytic (anxiety-reducing) medication for horses. [1][2] Veterinarians use acepromazine to help control animals who are difficult to handle, reduce anxiety during stressful situations, or support broader sedation protocols. [3][4]

This medication does not provide pain relief on its own. Instead, it helps make the horse calmer and less reactive so veterinary care, handling, transport, or other procedures can be performed more safely.

Depending on the situation, acepromazine may be used alone or combined with other sedatives, analgesics, or anesthetic drugs as part of a larger treatment plan.

Because acepromazine can affect blood pressure, coordination, body temperature, and penile retraction in male horses, it is not appropriate for every case.

The treating veterinarian will consider the horse’s health status, temperament, age, hydration, procedure, and risk factors before deciding whether acepromazine is suitable.

Acepromazine is a prescription-only medication and may only be legally obtained and administered under the direction of a licensed veterinarian.

 

illustration of acepromazine molecule

 

Available Forms

Acepromazine is available for veterinary use in both injectable and tablet forms. [4] The injectable medication is sterile and can be given into a vein, into the muscle, or under the skin. [3]

Tablets are approved for use in dogs, but equine veterinarians may prescribe them for horses in an off-label manner when appropriate. Owners may be instructed to mix tablets into the horse’s feed or dissolve them into a slurry for administration with an oral syringe.

Common veterinary brand names and product names include:

  • PromAce®
  • Atravet®
  • Aceprotabs

Brand availability and approved species vary by country, formulation, and manufacturer.

Enjoy $10 Off From Us

What's your top priority with your horse's health?

* Limit one per customer

Drug Class: Phenothiazines

Phenothiazines are a group of antipsychotic medications historically used to treat severe forms of mental illness in people. [5]

Acepromazine was originally used in human medicine for schizophrenia, but this use was discontinued because of side effects and poor efficacy. [6]

In veterinary medicine, acepromazine remains widely used for its sedative and anxiety-reducing effects. [6] Other phenothiazines include: [5]

  • Chlorpromazine
  • Fluphenazine
  • Prochlorperazine

Acepromazine is the only veterinary-approved phenothiazine currently available. [1][2]

Mechanism of Action

The precise mechanism of action for phenothiazines is unknown. [5] These drugs mainly act on dopamine receptors, reducing dopamine activity in the nervous system.

Dopamine is an excitatory neurotransmitter, meaning it increases the transmission of electrical signals through nerves. By reducing dopamine activity, acepromazine produces a calming effect that decreases movement and reactivity. [7]

Phenothiazines can also act on other types of receptors, causing effects beyond sedation. The most notable is acepromazine’s effect on alpha-1 receptors, which help regulate blood pressure. Acepromazine inhibits these receptors, leading to vasodilation and lower blood pressure. [7]

This effect can have significant consequences for horses that already have low blood pressure before administration.

Uses in Horses

Acepromazine is approved for use in controlling fractious animals (i.e., animals who are difficult or unsafe to handle) during examination, treatment, loading, or transportation. [3] It can also be used as a sole sedative for some procedures, including castration, skin biopsies, ocular procedures, or cast application. [3]

Many veterinarians primarily use acepromazine as part of their sedation protocols to help reduce anxiety and stress before sedation or anesthesia. For standing sedation, combining acepromazine with other sedatives provides more reliable sedation and may help reduce the amount of other sedative drugs required. [7]

Common reasons for using standing sedation include: [8]

For general anesthesia, using acepromazine can reduce the amount of inhaled anesthetic required. [7] It can also help improve recovery from anesthesia by delaying the time before the horse attempts to stand. [7]

Off-Label Uses

Injectable acepromazine products are approved for use in horses, including for controlling fractious animals during examination, treatment, loading, or transportation. [3] Oral acepromazine tablets, however, are only approved for use in dogs. [9]

Equine veterinarians may prescribe acepromazine tablets for horses in an off-label manner when they determine this use is appropriate. Off-label use is permitted in veterinary medicine when a veterinarian prescribes a drug outside the exact species, route, dose, or indication listed on the product label.

In these cases, the veterinarian is responsible for determining whether the medication is appropriate, providing dosing instructions, and monitoring for safety.

Administration Routes

Acepromazine is approved for multiple routes of administration in horses: [3]

  • Intravenous, or into a vein
  • Intramuscular, or into the muscle
  • Subcutaneous, or under the skin

Dosage

The dose of acepromazine used depends on the degree of sedation required and the horse’s level of anxiety. Horse owners should only give acepromazine by intramuscular or subcutaneous injection unless otherwise instructed by their veterinarian.

Intravenous injection carries a high risk if the drug is accidentally injected into an artery, which can cause immediate side effects such as seizures, collapse, and potentially death. [4] Owners should follow their veterinarian’s instructions closely and ask for a demonstration if they are unsure how to administer the medication safely.

The general dose for intramuscular or subcutaneous injection is 2 – 4 mg of drug per 100 lb of the horse’s body weight. [3] It can take 15 – 30 minutes for the drug to take effect after administration.

Medication dosages must be determined by a licensed veterinarian based on the individual horse’s condition. Incorrect dosing can cause serious harm. Always follow the prescription label exactly and never adjust a dose without professional direction.

Storage

Acepromazine should be protected from light and kept from freezing. Tablets should be stored in tight containers. [4]

Safety Warnings & Precautions

Acepromazine is generally safe when proper precautions are taken to reduce the risk of side effects and injury. Veterinarians monitor sedated horses carefully for changes in response, cardiovascular status, and procedure safety. They may also adjust the dose administered based on their findings.

Because acepromazine can lower blood pressure and affect coordination, owners should use the medication only as prescribed and avoid routine use without veterinary guidance.

Monitoring

Monitoring horses receiving acepromazine focuses on their level of sedation and whether the procedure can be performed safely. [4] Veterinarians may monitor the horse’s heart rate and rhythm to identify problems associated with low blood pressure.

Male horses should be checked for penile retraction, and the penis should be protected from injury if it remains dropped. [4] Continued monitoring is especially important when the horse is recovering from sedation or returning to normal activity.

Contraindications

Contraindications refer to circumstances where a drug should be avoided or used with caution.

Acepromazine can cause potent vasodilation, or widening of the blood vessels. Horses with cardiovascular disease or debilitation have a higher risk of hypotension, or low blood pressure, after receiving acepromazine.

Caution should be used in horses with: [4]

Historically, acepromazine was contraindicated in stallions and geldings. There is a risk of penile protrusion that can last for 2 hours or longer. [4] Injury to the penis during this time, or prolonged dropping of the penis, may cause swelling and permanent paralysis of the penis retractor muscle.

Current estimates suggest that the risk of permanent penile dysfunction is less than 1 in 10,000 cases. [10] Because more recent studies suggest complications are rare, many veterinarians now use acepromazine regularly in male horses. More caution may be warranted in breeding stallions, where penile dysfunction may cause reproductive loss. [7]

Side Effects

Most side effects of acepromazine administration are mild. Common side effects include: [4][7]

Of these, penile prolapse is the most significant side effect. Rarely, horses may be unable to retract their penis during and after administration of acepromazine. Having the penis dropped for several hours can result in edema and swelling, preventing retraction once the drug wears off completely.

Left untreated, this can cause permanent damage to the penis and may affect reproductive ability. Veterinarians may choose alternate sedatives in breeding stallions to reduce the risk of penile injury. [7]

Acute Toxicity (Overdose)

Overdose in horses has not been widely reported. [4]

In dogs and cats, signs of acepromazine overdose include: [4]

  • Extremely low blood pressure
  • Extreme sedation
  • Uncoordinated movement
  • Seizures
  • Recumbency

Typical management includes supportive veterinary care while the drug is metabolized by the body. [4] Most animals recover uneventfully with prompt veterinary intervention.

Intro to Equine Nutrition
Gain a deeper understanding of your horse's nutrition needs. This free introductory course provides a foundation for horse owners to learn how to balance your horse's diet.
Enroll Now
Introduction to Equine Nutrition Course - Mad Barn Academy

Drug Interactions

Phenothiazines act on many different receptors in the body, so the risk of potential drug interactions is high. A veterinarian should review the horse’s full medication history before acepromazine is administered.

Potential interactions include: [4]

  • Acetaminophen (Tylenol®): May increase the risk of low body temperature.
  • CNS depressants: Includes barbiturates, opioids, anesthetics, and other drugs that may cause more severe sedation.
  • Dopamine: May impair dopamine’s ability to raise blood pressure.
  • Epinephrine and ephedrine: Phenothiazines block alpha-adrenergic receptors, so concurrent use may result in vasodilation and increased heart rate.
  • Metoclopramide: May increase the risk of side effects from metoclopramide.
  • Organophosphate antiparasitics: May have a higher risk of side effects when given with acepromazine.
  • Phenytoin: Phenothiazines may decrease metabolism of this medication.
  • Procaine: Acepromazine may enhance activity of this medication.
  • Propranolol: Blood levels of both drugs may increase if given concurrently.
  • Quinidine: May cause additional cardiac side effects when given concurrently.

Drugs that reduce gastrointestinal absorption, such as antacids or kaolin/pectin, may interfere with the absorption of oral acepromazine. [4]

If a drug interaction is not listed by the manufacturer, it does not mean no interaction exists. Always notify your veterinarian about all medications and supplements your horse has had before starting treatment with a new medication.

Regulatory Status & Legal Considerations

Acepromazine injectable formulations are approved by Health Canada and the FDA for use in horses. [1][2] The primary indication for use is controlling fractious animals. Veterinarians may also use the medication to facilitate procedures such as castration, skin biopsy, and other veterinary care. [3]

The oral tablet formulation of acepromazine is currently labelled only for use in dogs. However, veterinarians may use this formulation in horses in an off-label manner.

Status in Competition

Acepromazine reduces anxiety and responsiveness to external stimuli, which may give a horse an unfair advantage by making it easier to handle or less reactive in stressful competition environments.

As a result, acepromazine is prohibited during competition under the rules of the Fédération Équestre Internationale (FEI). [11]

Horse owners should always confirm the rules of their competition organization before any medication is administered. Competition requirements can vary by organization, discipline, and jurisdiction.

Frequently Asked Questions

Here are some frequently asked questions about acepromazine for horses:

Summary

Acepromazine is a widely used equine tranquilizer that helps reduce anxiety and improve handling during veterinary procedures, transportation, and other stressful situations.

  • Acepromazine is a sedative and tranquilizer, not a pain medication
  • It is commonly used before veterinary procedures, for trailering, and to help manage anxious or excitable horses
  • The medication can lower blood pressure and should be used cautiously in horses with cardiovascular disease, dehydration, or shock
  • Side effects are generally mild but may include prolonged sedation, low blood pressure, and, rarely, penile prolapse in geldings and stallions
  • Always follow your veterinarian's dosing instructions and competition withdrawal guidelines to ensure the medication is used safely and legally
Related Articles
Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. Drug Product Database. Health Canada. 2026.
  2. Animal Drugs @ FDA. Food and Drug Administration. 2026.
  3. PromAce® Injectable(Acepromazine Maleate Injection). Boehringer Ingelheim. 2019.
  4. Budde. J. A. et al. Eds. Plumb's Veterinary Drug Handbook. 10th edition. DEducational Concepts, LLC, dba VetMedux, Tulsa, OK. 2023.
  5. Kidron. A. and Nguyen. H. Phenothiazine. StatPearls. 2026.
  6. Algren. D. A. and Ashworth. A. Acute Acepromazine Overdose: Clinical Effects and Toxicokinetic Evaluation. Journal of Medical Toxicology. 2015.
  7. Doherty. T. et al. Eds. Manual of Equine Anesthesia and Analgesia. 1st ed. Wiley. 2022.
  8. Reed. S. M. et al. Equine Internal Medicine. 3rd ed. Saunders Elsevier. 2010.
  9. PromAce® Tablets (Acepromazine Maleate Tablets). Boehringer Ingelheim. 2025.
  10. Driessen. B. et al. Contemporary Use of Acepromazine in the Anaesthetic Management of Male Horses and Ponies: A Retrospective Study and Opinion Poll. Equine Veterinary Journal. 2011. View Summary
  11. 2026 Equine Prohibited Substances List. 2026.