Acepromazine

Overview

Acepromazine (PromAce®) is a phenothiazine sedative/tranquilizer widely used in veterinary medicine across multiple species. It produces dose-dependent tranquilization and muscle relaxation without providing analgesia and is commonly incorporated into balanced anesthetic or analgesic protocols.

Compared with many other sedatives, acepromazine has a relatively slow onset of action, with effects beginning within approximately 15 minutes and reaching full effect by 30 minutes. Its duration of action is prolonged, typically lasting 3 to 4 hours and up to 6–8 hours in some animals. The drug does not have a reversal agent, and termination of effect depends on hepatic metabolism.

Mechanism of Action (MOA): Acepromazine is a phenothiazine neuroleptic that primarily blocks postsynaptic dopamine receptors in the central nervous system, resulting in CNS depression and tranquilization. It also produces alpha-1 adrenergic blockade, leading to peripheral vasodilation and potential hypotension. Additional pharmacologic actions include antihistaminic, antiemetic, antispasmodic, muscle relaxant, and mild antiarrhythmic effects.

Indications

Acepromazine is FDA-approved for use in dogs, cats, and horses and is one of the most commonly used sedatives/tranquilizers in veterinary practice. It is primarily indicated for use in stable patients where tranquilization, chemical restraint, or preanesthetic sedation is required.

• Sedation and tranquilization: Used to control excitable, anxious, or fractious animals during handling, transport, grooming, diagnostic procedures, and minor interventions.
• Preanesthetic medication: Commonly administered as part of balanced anesthetic protocols to provide tranquilization and muscle relaxation and to reduce induction and inhalant anesthetic dose requirements.
• Antiemetic use: Labeled for prevention and control of vomiting associated with motion sickness in dogs and cats.
• Adjunct to analgesic protocols: Combined with opioid analgesics to enhance sedation and prolong analgesic effects (neuroleptanalgesia), despite having no intrinsic analgesic activity.
• Behavioral and stress-related situations: Oral formulations are commonly used to blunt responses to fear- or stress-inducing stimuli such as travel, fireworks, or thunderstorms, although true anxiolysis is inconsistent. Not recommended for using acepromazine as a first choice for stress in hospitalized animals. Specific anxiolytic agents (e.g., trazodone, gabapentin) are preferred for true anxiety with fewer adverse effects.
• Urethral obstruction (adjunctive use in male cats): Used as part of a multimodal protocol to reduce urethral spasm and facilitate resolution without catheterization in selected stable cases.
• Use in horses: Indicated as an aid in controlling fractious animals, as a preanesthetic sedative, and as an adjunctive treatment in conditions such as laminitis to improve peripheral blood flow.
• Other species (extra-label): Used as a tranquilizer in cattle, sheep, goats, swine, rabbits, and other small mammals when clinically appropriate.

Dosage (Reference)

Dog

In dogs, the FDA-labeled doses of acepromazine are considered excessively high by most clinicians. Clinical practice strongly favors lower, extra-label dosing, which provides effective tranquilization with a substantially lower risk of hypotension and prolonged sedation.

Cat

In cats, acepromazine should be used cautiously. Extra-label low doses are preferred, as labeled doses are frequently excessive and associated with hypotension and hypoventilation. Only stable, non-obtunded cats should receive the drug.

Warnings & Precautions

Acepromazine is a long-acting tranquilizer with no reversal agent. Although it has minimal effects on respiratory function, its cardiovascular, thermoregulatory, and hematologic effects require careful patient selection, dose adjustment, and close monitoring.

• No reversal agent: Sedative effects depend on hepatic metabolism for termination. Prolonged or exaggerated sedation may occur in geriatric, pediatric, or hepatic-impaired patients.
• Hypotension: Alpha-1 adrenergic blockade causes peripheral vasodilation and may result in clinically significant hypotension, with or without reflex tachycardia. Avoid use or reduce the dose in hypovolemic, dehydrated, hypotensive, or shocked patients.
• Cardiac disease: Use cautiously in patients with mild cardiac disease that may not tolerate reduced blood pressure. Avoid use in animals with significant cardiac disease, severe hypotension, or shock.
• Hepatic dysfunction: Use at the low end of the dose range in animals with mild to moderate hepatic disease and avoid use in cases of severe hepatic dysfunction.
• MDR1 mutation (dogs): Dogs with the MDR1 (ABCB1) gene mutation may experience prolonged or exaggerated sedation. Avoid use when possible; if necessary, substantially reduce the dose.
• Aggressive or fractious dogs: When used alone, acepromazine may increase startle responses and worsen aggression or CNS excitation. Use cautiously and preferably in combination with other agents.
• Pain management: Provides no analgesia. Painful animals must receive appropriate analgesics to prevent masking of pain-related behaviors and negative welfare outcomes.
• Anemia and coagulopathies: Causes splenic sequestration of red blood cells and may reduce platelet aggregation. Use cautiously in anemic, thrombocytopenic, or coagulopathic patients.
• Organophosphate or procaine exposure: Contraindicated in animals receiving or recently exposed to organophosphate insecticides or procaine due to potentiation of toxicity.
• IV administration: Must be administered slowly to reduce the risk of profound hypotension, cardiovascular collapse, or paradoxical CNS effects. Do not administer intra-arterially.
• Thermoregulation: CNS depression may impair temperature regulation and lead to hypothermia, particularly in small, debilitated, or anesthetized patients.
• Breed-specific concerns: Boxers and some brachycephalic dogs may be prone to syncope associated with sinoatrial block; use low doses and monitor closely.
• Use in cats: Use cautiously due to the risk of hypotension and hypoventilation. Avoid use in obtunded or unstable cats.
• Pediatric (<3 months): Pediatric (<3 months): Avoid use in puppies and kittens younger than 3 months of age due to increase risk of prolonged sedation.
• Seizures / epilepsy: Seizures / epilepsy: The historical concern that acepromazine lowers the seizure threshold has been disproved. Recent research suggests acepromazine may actually have some anticonvulsant activity and did not cause seizures even in high-risk dogs. Use is no longer contraindicated in animals with a seizure history

Drug Interactions

Clinically relevant drug interactions with acepromazine are primarily related to additive central nervous system depression, cardiovascular effects, or interference with dopamine-mediated and cardiac conduction pathways. When used concurrently with other medications, dose adjustment and increased monitoring may be required.

• Opioids (e.g., morphine, fentanyl, buprenorphine): Additive sedation and hypotension may occur; reduce the acepromazine dose when used in combination.
• Alpha-2 agonists (e.g., dexmedetomidine, xylazine): Increased risk of profound CNS depression, bradycardia, and hypotension when combined.
• General anesthetic agents (e.g., alfaxalone, ketamine, isoflurane): Additive CNS and cardiovascular depression; anesthetic dose requirements are reduced.
• Benzodiazepines and barbiturates: Enhanced CNS depression may occur; close monitoring is recommended.
• Antipyretics and NSAIDs: Concurrent use may increase the risk of hypothermia; monitor body temperature.
• Dopamine agonists (e.g., bromocriptine, cabergoline, pergolide): Mutual antagonism may reduce therapeutic effectiveness of both drugs; avoid combination.
• Epinephrine: Contraindicated for treatment of acepromazine-induced hypotension due to the risk of epinephrine reversal and worsening hypotension.
• Hypotensive agents (e.g., ACE inhibitors, calcium channel blockers): Increased risk of clinically significant hypotension; blood pressure monitoring is recommended.
• QT-prolonging drugs (e.g., cisapride, erythromycin, sotalol): Concurrent use may increase the risk of cardiac arrhythmias and QT prolongation and should be avoided.
• Antacids, sucralfate, and antidiarrheal mixtures: May reduce oral absorption of acepromazine; administer acepromazine at least 2 hours before these agents.
• Organophosphate compounds: Potentiation of toxicity may occur; concurrent or recent exposure is contraindicated.
• Anticholinergic agents (e.g., atropine, glycopyrrolate): May increase anticholinergic adverse effects such as dry mucous membranes and urinary retention.
• Metoclopramide: Increased risk of extrapyramidal effects and reduced prokinetic efficacy.
• Propranolol and quinidine: Potential for increased drug concentrations and additive cardiac depression.

Side Effects & Overdose

Side Effects

Adverse effects of acepromazine are dose-dependent and are more likely to occur with higher doses, rapid IV administration, or in compromised patients. Most effects are related to cardiovascular depression, central nervous system actions, and alterations in thermoregulation.

• Hypotension: Common and clinically significant due to peripheral vasodilation from alpha-1 adrenergic blockade; may be accompanied by reflex tachycardia.
• Prolonged or exaggerated sedation: More likely in geriatric, pediatric, debilitated, or hepatic-impaired animals.
• Splenic sequestration of red blood cells: Transient decreases in PCV may occur and can exacerbate clinical signs in anemic patients.
• CNS effects: Paradoxical excitement, restlessness, agitation, or worsening aggression may be observed in some animals.
• Hypothermia: Resulting from CNS depression and impaired thermoregulation.
• Reduced tear production: Reported particularly in cats and rabbits; may contribute to ocular dryness.
• Prolapse of the nictitating membrane: Common and benign finding in many species following administration.
• Penile protrusion (horses): Dose-related and usually transient; permanent dysfunction is rare but possible with prolonged protrusion.
• Injection site discomfort: Transient pain may occur following IM administration; SC administration may be less painful.

Overdose

Acepromazine has a relatively wide safety margin; however, overdose or supratherapeutic dosing can result in severe cardiovascular and neurologic complications, particularly after large oral ingestions or rapid parenteral administration.

• Severe hypotension: Due to profound peripheral vasodilation and central depression; may progress to cardiovascular collapse.
• CNS depression: Marked sedation, stupor, or coma; extrapyramidal signs such as tremors, rigidity, or excitement may occur due to dopamine blockade.
• Respiratory compromise: Usually mild but may occur secondary to severe CNS depression.
• Pulmonary edema and hyperemia of internal organs: Reported with massive overdoses.
• Management: Treatment is primarily supportive and includes cardiovascular monitoring, IV fluid therapy, temperature support, and oxygen as needed.
• Vasopressors: Alpha-adrenergic agents (e.g., norepinephrine, phenylephrine) may be used if hypotension persists despite adequate fluid therapy.
• Epinephrine: Contraindicated due to the risk of epinephrine reversal and worsening hypotension.
• Seizure control: Benzodiazepines may be administered if seizures or severe CNS excitation occur.

Key Notes

Practical clinical considerations that support appropriate use of acepromazine in everyday veterinary practice, focusing on points not emphasized elsewhere:

• Predictability vs depth: Acepromazine provides reliable tranquilization but does not reliably produce deep sedation; the clinical effect may be insufficient for painful or highly stimulating procedures when used alone.
• Arousal override: Sedative effects can be partially or completely overridden by strong sensory stimuli (noise, handling, pain), making the drug unreliable as a sole chemical restraint agent.
• Interindividual variability: Response varies widely between patients and breeds; dosing should be titrated to clinical effect rather than relying strictly on body weight.
• Anesthetic-sparing effect: Patients premedicated with acepromazine often require lower doses of induction and inhalant anesthetics, contributing to smoother anesthetic induction and recovery.
• Use in airway disease: Minimal impact on blood gas values makes acepromazine a useful option in selected patients with upper airway obstruction where other sedatives may pose higher risk.
• Behavioral limitations: Sedation should not be mistaken for anxiolysis; acepromazine may suppress outward responses without addressing underlying anxiety or fear.
• Procedure planning: The relatively long duration of action can be beneficial for prolonged procedures or recovery periods but may delay same-day discharge.
• Client communication: Owners should be advised that animals may appear calm yet remain responsive and may startle unexpectedly during the sedative period.