Ace is the common nickname for acepromazine, a tranquilizer widely used in horses for sedation, anxiety reduction, and certain medical conditions. It works by blocking dopamine receptors in the brain, which calms the horse without putting it to sleep. Ace is one of the most frequently used sedatives in equine medicine, available in both injectable and oral forms.
How Ace Works in the Body
Acepromazine belongs to a class of drugs called phenothiazine tranquilizers. Its primary job is blocking dopamine signaling in the central nervous system, which produces a visible calming effect: the horse’s head typically drops, its lower lip relaxes, and it becomes less reactive to stimuli. The drug also blocks adrenaline-type receptors throughout the body, which causes blood vessels to relax and widen. This is why a horse on Ace often looks noticeably “droopy” and may have a slightly flushed or warm appearance.
That blood vessel relaxation is actually useful in some medical situations. When given intravenously, Ace appreciably increases blood flow through the arteries and tissues of the hoof. This makes it a treatment option for horses with laminitis, where restricted blood flow to the hoof is a central problem.
Common Uses
Most horse owners encounter Ace in one of a few situations:
- Veterinary procedures. Clipping, dental work, wound care, and other hands-on tasks where a calmer horse makes the job safer for everyone.
- Trailering and travel. Horses that become anxious or dangerous during loading or transport are sometimes given Ace beforehand.
- Anxiety and behavioral management. Ace has been used as a behavior-modifying agent in horses with persistent anxiety.
- Laminitis treatment. The drug’s ability to increase digital blood flow makes it part of some laminitis management plans.
- Pre-anesthesia. Veterinarians frequently use Ace as a first step before general anesthesia for surgery, helping the horse transition more smoothly into deeper sedation.
Onset, Duration, and How It’s Given
Ace can be given by injection (into the muscle or vein) or orally. The route matters because it changes how quickly the drug kicks in and how long it lasts.
When injected intravenously, the effects begin within minutes. The drug’s half-life through this route is about 2.5 hours, meaning the peak sedation wears off relatively quickly, though some residual drowsiness can linger. Intramuscular injection is slightly slower to take effect but follows a similar timeline.
Oral Ace is absorbed rapidly, reaching peak blood levels in roughly 25 minutes. However, its elimination is notably slower by mouth, with a half-life of about 6 hours compared to 2.6 hours after IV injection. This means oral dosing produces a longer, more gradual sedation window. Research has shown that an oral dose of 0.5 mg per kilogram of body weight produces sedation similar to an IV dose of 0.1 mg per kilogram, so the oral dose needs to be considerably higher to achieve the same effect.
Typical Dosage Range
For intramuscular or intravenous injection, the standard dosage range is 0.03 to 0.10 mg per kilogram of body weight. For a typical 500 kg (1,100 lb) horse, that translates to roughly 15 to 50 mg total. Your veterinarian will choose a dose based on the horse’s temperament, the level of sedation needed, and whether other drugs are being used alongside it. IV injections are given slowly to reduce the risk of a sudden drop in blood pressure.
It’s worth noting that individual horses vary widely in their response to Ace. Some become deeply sedated at low doses while others seem barely affected. Draft breeds and hot-blooded breeds can respond quite differently, so dosing isn’t a one-size-fits-all calculation.
Side Effects and Blood Pressure
The most significant physiological effect of Ace beyond sedation is a drop in blood pressure. By blocking adrenaline receptors in blood vessel walls, the drug causes widespread vasodilation. In a healthy, well-hydrated horse, this blood pressure drop is generally mild and without major consequences. But in a horse that is dehydrated, anemic, or already has low blood volume, the same effect can become dangerous, potentially causing a hypotensive crisis and collapse.
Other side effects include reduced ability to regulate body temperature (important in very hot or cold weather) and mild changes in heart rate. Because the drug lowers blood pressure so reliably, veterinarians monitor horses carefully when Ace is combined with other sedatives or anesthetic agents.
Penile Prolapse in Males
One side effect that gets a lot of attention among horse owners is penile prolapse in stallions and geldings. Ace causes the muscles that retract the penis to relax, and the penis may hang down visibly for hours after administration. A retrospective study of nearly 600 male horses found that about 2.4% experienced penile prolapse lasting between 30 minutes and 4 hours. In one stallion (0.2% of cases), prolapse lasted between 12 and 18 hours.
The fear that Ace causes permanent damage to stallions is widespread but largely overstated. The same study estimated the risk of permanent penile dysfunction at 1 in 10,000 cases or fewer, and the researchers concluded this extremely low risk does not justify avoiding Ace in intact males compared to geldings or mares. That said, many veterinarians still use lower doses in breeding stallions out of an abundance of caution, particularly if the horse’s reproductive career is a priority.
When Ace Should Not Be Used
Ace is contraindicated in several situations:
- Dehydrated, anemic, or shocky horses. The blood pressure drop can push an already compromised horse into a crisis.
- Liver, kidney, or heart disease. The drug is processed through the liver and affects cardiovascular function, making it risky in horses with organ dysfunction.
- History of seizures. Ace lowers the seizure threshold and is not recommended for horses with epilepsy or a history of fainting from heart rhythm abnormalities.
- Organophosphate exposure. Ace amplifies the toxicity of organophosphate compounds, which are found in some dewormers, insecticides, and flea products. It should never be used to control tremors from organophosphate poisoning or given alongside these products.
- Debilitated or very old horses. These animals are more sensitive to the cardiovascular effects and may not tolerate even standard doses well.
Ace and Competition Rules
Acepromazine is a prohibited substance in competitive horse sports. Under United States Equestrian Federation guidelines, horses must be withdrawn from competition for at least 7 days after receiving Ace or other short-acting tranquilizers like detomidine and xylazine. The same general principle applies under FEI (international) regulations for competitions held in the United States. These detection windows exist because even after the visible sedation has worn off, drug metabolites remain in the horse’s system and will show up on testing.
If you compete, keep detailed records of any Ace administration and plan your show schedule accordingly. The 7-day guideline is a minimum, and some horses may metabolize the drug more slowly depending on their liver function and overall health.
What Ace Does Not Do
One common misconception is that Ace provides pain relief. It doesn’t. Ace is a tranquilizer, not an analgesic. A horse on Ace may appear calm and unbothered, but it still feels pain normally. This distinction matters because using Ace alone for painful procedures like wound suturing or joint injections would leave the horse sedated but still in pain, which can lead to sudden, dangerous reactions. For procedures involving pain, veterinarians typically combine Ace with an actual painkiller or use a different sedation protocol altogether.
Ace also does not reliably sedate every horse to the same degree. Horses that are already highly agitated or fearful may override the drug’s effects, especially at lower doses. In emergency or high-stress situations, veterinarians often reach for faster-acting, more potent sedatives instead.