Acepromozine,
hydroxyethylpromazine sulfoxide

Uses & side effects

Acepromazine (commonly called "Ace" or PromAce) is a phenothiazine derivative sedative and tranquilizer used in horses to alleviate anxiety, facilitate safe handling, and act as a pre-anesthetic agent.Unlike profound hypnotics, Ace is a mild central nervous system depressant that calms a horse without causing unsteadiness or deep sleep.Primary Clinical UsesFacilitating Routine Procedures: It is widely used to quiet horses for non-painful but stressful tasks like clipping, shoeing, veterinary examinations, loading into trailers, or standing for dental work.Controlled Stall Rest & Rehabilitation: For high-strung performance horses recovering from soft-tissue injuries (like tendon or ligament tears), Ace is given daily to prevent them from explosive movements or running in small paddocks.Pre-Anesthetic Protocol: It is routinely paired with opioids (like butorphanol) or alpha-2 agonists (like xylazine) prior to general anesthesia. It smoothens the induction phase and reduces the total amount of riskier anesthetic drugs needed.Vasodilation for Laminitis: Because Ace blocks alpha-1 receptors, it opens peripheral blood vessels. Some veterinarians utilize low doses to improve microcirculation and blood flow to the hooves during acute laminitis episodes.Administration and FormulationsInjectable (IV / IM): Administered at a standard clinical dose of 0.02 to 0.05 mg/kg. It takes 15 to 20 minutes to take full effect via the intravenous route, and roughly 30 to 45 minutes intramuscularly.Oral Pastes / Gels: Administered directly into the horse's cheek or under the tongue. It has a slower onset time (45 to 60 minutes) but provides a more sustained, longer-lasting calming effect.Key Clinical Rules for SuccessThe "Quiet Window" Rule: Acepromazine does not block adrenaline. If a horse is already highly excited, terrified, or aggressive before you give the drug, the surging adrenaline will completely override the sedation, rendering the dose entirely ineffective. You must medicate the horse in a calm, quiet environment and leave it alone for 20 minutes while the drug kicks in.No Analgesic Properties: Ace provides zero pain relief. If you attempt a painful procedure (such as suturing a wound) using only Ace, the horse will still feel the pain fully and may kick or strike unpredictably. For painful tasks, it must be combined with a true analgesic.Severe Risks and Side EffectsPenile Prolapse & Priapism: Ace causes profound relaxation of the retractor penis muscle. In rare cases, this relaxation can progress to permanent paralysis (priapism), requiring emergency surgical intervention or amputation. Because of this catastrophic risk, most veterinarians will not administer Ace to intact breeding stallions.Hypotension (Blood Pressure Drop): By causing systemic vasodilation, Ace triggers a sharp drop in blood pressure. It also forces blood to pool in the spleen, resulting in a temporary 20% to 30% drop in packed cell volume (hematocrit). It should never be given to horses in shock, those suffering from severe dehydration, or those experiencing acute blood loss.

Product-specific side effects await source review.

Drug interactions

Absorption and BioavailabilityThe timeline for absorption and peak plasma concentration depends heavily on the route of administration:Intravenous (IV): Achieves immediate bioavailability, with plasma concentrations peaking within 5 minutes.Intramuscular (IM): Rapidly absorbed, reaching peak plasma concentration around 30 minutes post-injection.Oral (PO) and Sublingual (SL): Oral gels and pastes are absorbed quickly through the mucous membranes, peaking in the bloodstream between 24 to 50 minutes.Distribution and Protein BindingAcepromazine is highly lipophilic (fat-soluble), which heavily dictates how it moves through equine tissue:Volume of Distribution (\(V_{d}\)): Highly distributed throughout body tissues (\(V_d \approx 6.6 \text{ L/kg}\)), meaning very little parent drug remains floating freely in the bloodstream.Protein Binding: It binds extensively to equine plasma proteins at a rate greater than 99%.Blood Partitioning: In whole blood, the drug splits relatively evenly: roughly 46% partitioning into the plasma phase and 54% binding within the erythrocyte (red blood cell) phase.Metabolism and EliminationThe liver and blood cells rapidly break down the parent drug into structurally altered compounds:Hepatic and RBC Metabolism: Acepromazine undergoes fast, erratic metabolism primarily via the liver. In vitro data from PubMed equine studies also demonstrate that red blood cells actively metabolize the drug.Primary Metabolites: The parent drug breaks down into two major metabolites: 2-(1-hydroxyethyl) promazine (HEP) and 2-(1-hydroxyethyl) promazine sulfoxide (HEPS).Clearance: Total body clearance is high, estimated between 2.5 and 3.0 L/kg/hr.Pharmacokinetic vs. Pharmacodynamic DisconnectA unique hallmark of acepromazine in horses is that clinical and physiological effects do not correlate with blood concentrations.Central Nervous System (CNS) sedation peaks at 20 minutes, while severe hemodynamic changes (such as drop in blood pressure and a 20% decline in packed cell volume/hematocrit) peak at 100 minutes.These profound cardiovascular and sedative effects persist long after the parent drug drops below detectable levels in the plasma. This lag suggests that the active metabolites (HEP/HEPS) continue driving the drug's clinical action, or that complex tissue-compartment shifting is at play.Forensic Detection WindowsBecause acepromazine is a regulated, prohibited substance in competition, its rapid elimination requires specific forensic tracking according to the Racing Medication & Testing Consortium (RMTC):Parent Drug: Unmodified acepromazine is cleared quickly and is rarely quantifiable in plasma past 3 to 8 hours.HEPS Marker: Regulatory labs target the HEPS metabolite because of its prolonged window. HEPS remains detectable for up to 24 hours in plasma and up to 120 to 144 hours (5 to 6 days) in urine.

Contraindications