Acepromazine
**Acepromazine** is a classic, widely utilized phenothiazine neuroleptic agent in veterinary medicine, primarily used for its reliable tranquilizing and sedative properties. **Key Clinical Points:** * **No Analgesia:** Acepromazine provides **zero pain relief**. It must be combined with appropriate analgesics (e.g., opioids) for painful procedures (neuroleptanalgesia). * **Cardiovascular Effects:** It is a potent vasodilator due to alpha-1 adrenergic blockade, frequently causing **hypotension**. It should be avoided in hypovolemic, shocked, or hemodynamically unstable patients. * **Breed Sensitivities:** Giant breeds and sight hounds (e.g., Greyhounds) are highly sensitive. Dogs with the **MDR1 (ABCB1) gene mutation** (e.g., Collies, Australian Shepherds) require significant dose reductions (25-50%). * **Equine Considerations:** May cause prolonged penile protrusion or priapism in stallions, which can lead to permanent paralysis of the retractor penis muscle. While historically used for travel anxiety or noise phobias, modern veterinary behaviorists often discourage its use as a monotherapy for these conditions, as it may sedate the animal without reducing the underlying fear or panic, potentially increasing noise sensitivity.
Mechanism: Acepromazine exerts its effects through multiple receptor antagonisms: * **Dopamine (D2) Receptor Antagonism:** Blocks postsynaptic D2 receptors in the CNS (specifically the basal ganglia and limbic system) โ depresses the reticular activating system โ produces **sedation and tranquilization**. * **Alpha-1 Adrenergic Antagonism:** Blocks peripheral alpha-1 receptors โ causes peripheral vasodilation โ leads to **hypotension** and secondary hypothermia. * **Additional Blockade:** Possesses varying degrees of **anticholinergic** (muscarinic), **antihistaminic** (H1), and **antispasmodic** effects. * **Antiemetic Effect:** Blocks dopamine receptors in the Chemoreceptor Trigger Zone (CRTZ) of the medulla. > **Clinical Pearl:** Because of its alpha-blocking properties, administering epinephrine to an acepromazine-induced hypotensive patient can cause **"epinephrine reversal"** (unopposed beta-2 vasodilation), worsening the hypotension. Alpha-agonists like phenylephrine are preferred for treating severe acepromazine-induced hypotension.
Dosing by species
- Sedation ยท 0.05-0.1 mg/kg IM ยท IM
- Sedation ยท 0.05-0.1 mg/kg IM ยท IM
- Labeled dose ยท 0.55-2.2 mg/kg PO or 0.55-1.1 mg/kg IV, IM or SC ยท PO, IV, IM, SC ยท Considered by many clinicians to be 10 times greater than necessary.
- Restraint/sedation ยท 0.025-0.2 mg/kg IV; maximum of 3 mg or 0.1-0.25 mg/kg IM ยท IV, IM
- Preanesthetic ยท 0.1-0.2 mg/kg IV or IM; maximum of 3 mg; 0.05-1 mg/kg IV, IM or SC ยท IV, IM, SC
- To reduce anxiety in the painful patient (not a substitute for analgesia) ยท 0.05 mg/kg IM, IV or SC; do not exceed 1 mg total dose ยท IM, IV, SC
- Premedication ยท 0.03-0.05 mg/kg IM or 1-3 mg/kg PO ยท IM, PO ยท Give at least one hour prior to surgery (PO not as reliable).
- As a premedicant with morphine ยท acepromazine 0.05 mg/kg IM; morphine 0.5 mg/kg IM ยท IM
- Sedation and premedication (Non-Boxers) ยท 0.01-0.02 mg/kg ยท IV ยท single dose ยท up to 6 hours ยท Administer slowly. Generally given as part of a combination with opioids.
- Sedation and premedication (Non-Boxers) ยท 0.01-0.05 mg/kg ยท IM/SC ยท single dose ยท up to 6 hours ยท Onset of sedation is 20-30 minutes after IM administration.
Routes of administration
Contraindications
- Significant cardiac disease
- Hypovolemia, hypotension, or shock
- Tetanus or strychnine intoxication
- Intra-arterial injection in horses (can cause severe CNS excitement, seizures, death)
- Use with extreme caution in very young, geriatric, or debilitated animals
- Avoid in racing animals within 4 days of a race
Adverse effects
- Hypotension and cardiovascular collapse
- Bradycardia (vagally mediated) or reflex tachycardia
- Hypothermia or hyperthermia
- Penile protrusion/prolapse in large animals (especially stallions)
- Prolapse of the membrana nictitans (third eyelid)
- Decreased tear production (especially in cats)
- Paradoxical excitement, restlessness, or aggression
- Transient pain at IM injection sites
- Decreased hematocrit (due to splenic sequestration of RBCs)
Drug interactions
- Acetaminophen ยท Possible increased risk for hypothermia
- Antacids ยท May cause reduced GI absorption of oral phenothiazines
- Antidiarrheal mixtures (Kaolin/pectin, bismuth) ยท May cause reduced GI absorption of oral phenothiazines
- CNS Depressant Agents (barbiturates, narcotics, anesthetics) ยท May cause additive CNS depression if used with acepromazine
- Dopamine ยท Acepromazine may impair the vasopressive action of dopamine
- Emetics ยท Acepromazine may reduce the effectiveness of emetics
- Epinephrine, Ephedrine ยท Concomitant use can lead to unopposed beta-activity causing vasodilation and increased cardiac rate (epinephrine reversal)
- Metoclopramide ยท May increase risks for extrapyramidal adverse effects
- Opiates ยท May enhance hypotensive effects; dosages of acepromazine are generally reduced when used with an opiate
- Organophosphate Agents ยท Effects may be potentiated; do not give within one month of worming with these agents
- Phenytoin ยท Metabolism may be decreased if given concurrently
- Procaine ยท Activity may be enhanced by phenothiazines
Monitoring
- Cardiac rate, rhythm, and blood pressure (especially in compromised patients)
- Degree of tranquilization and sedation
- Male horses: Monitor to ensure the penis retracts and is not injured
- Body temperature (especially if ambient temperature is very hot or cold)
Overdose
The LD50 in mice is 61 mg/kg IV and 257 mg/kg PO. Dogs have survived oral dosages up to 220 mg/kg, but overdoses can cause serious **hypotension, CNS depression, pulmonary edema, and hyperemia**. **Clinical Signs of Toxicity:** * **Dogs:** Ataxia, sedation, lethargy, depression, protrusion of the third eyelid, somnolence, bradycardia, and recumbency. * **Cats:** Sedation, ataxia, lethargy, protrusion of the third eyelid, and depression. **Treatment:** * Because of relatively low toxicity, most overdoses are handled by monitoring and symptomatic treatment. * Massive oral overdoses should be treated by emptying the gut if possible. * **Hypotension:** Should *not* be treated initially with fluids. If fluids fail to maintain BP, use alpha-adrenergic pressor agents (e.g., phenylephrine). *Note: Avoid epinephrine due to the risk of "epinephrine reversal" causing further vasodilation.* * **Seizures:** May be controlled with barbiturates or diazepam. * **CNS Depression:** Doxapram has been suggested as an antagonist to the CNS depressant effects.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.