Chlorpromazine

Chlorpromazine is a phenothiazine tranquilizer with centrally acting antiemetic properties. In small-animal medicine, it has been used for selected causes of vomiting and occasionally for sedation or preanesthetic medication. Its clinical use is largely empirical, and more selective antiemetics and other tranquilizers have replaced it in many settings.

Group

Phenothiazine antiemetic and tranquilizer

Mechanism of Action

Chlorpromazine antagonizes dopamine receptors in the central nervous system, including receptors involved in signaling from the chemoreceptor trigger zone to the vomiting center. This suppresses some centrally mediated emetic stimuli. Dopamine antagonism in other brain pathways contributes to tranquilization, while alpha-adrenergic, histaminergic and muscarinic receptor blockade accounts for additional peripheral and central effects.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Vomiting	0.5 mg/kg	IM, SC, IV	Every 6–8 h	Dilute IV product to no more than 1 mg/mL and administer slowly; monitor blood pressure.
Chemotherapy-associated vomiting	2 mg/kg	SC	Every 3 h	Higher-dose alternative described in the administration discussion; evidence is empirical.
Vomiting	0.2–0.4 mg/kg	SC, IM	Every 8 h
Fixed-dose oral antiemetic treatment	Reference only	PO		4–8 mg per dog orally every 12 hours, not exceeding 0.5 mg/kg per dose. Current schema cannot apply the per-kg ceiling to a fixed amount.
Historical tetanus muscle-relaxant use	Reference only	IM		2 mg/kg IM twice daily is described for tetanus, but the same monograph cautions against phenothiazines with tetanus or strychnine intoxication because of extrapyramidal effects. Retain the conflict rather than enable a routine regimen.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Vomiting	0.2–0.4 mg/kg	IM, SC	Every 6–8 h
Higher-dose antiemetic treatment	0.5 mg/kg	IM, SC	Every 8 h
Chemotherapy-associated vomiting	2 mg/kg	SC	Every 3 h	Higher-dose alternative described in the administration discussion; evidence is empirical.
Vomiting	0.5 mg/kg	IV, IM, SC	Every 6–8 h	Dilute IV product to no more than 1 mg/mL and administer slowly; monitor blood pressure.
Vomiting	2 mg/animal	PO	Every 12 h
Premedication	Reference only	IM		Up to 1.1 mg/kg IM 1–1.5 hours before surgery; no lower endpoint is supplied, so an artificial range is not generated.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Sedation	0.22 mg/kg	IV	Once
Sedation	1.1 mg/kg	IM	Once

Sheep

Use / stage	Dose	Route	Schedule
Sedation	0.55 mg/kg	IV	Once
Sedation	2.2 mg/kg	IM	Once
Sedation	0.55–4.4 mg/kg	IV
Sedation	2.2–6.6 mg/kg	IM

Goats

Use / stage	Dose	Route	Schedule
Sedation	0.55 mg/kg	IV	Once
Sedation	2.2 mg/kg	IM	Once
Sedation	0.55–4.4 mg/kg	IV
Sedation	2.2–6.6 mg/kg	IM
Sedation	2–3.5 mg/kg	IV	Every 5–6 h

Pigs

Use / stage	Dose	Route	Schedule	Instructions
Sedation	0.5 mg/kg	IM	Once
Premedication	1 mg/kg	IM
Sedation	0.55–3.3 mg/kg	IV
Premedication	2–4 mg/kg	IM
Restraint	1.1 mg/kg	IM		Peak effect around 45–60 minutes.

Indications

Empirical control of centrally mediated vomiting in dogs and cats, including selected drug-associated and gastrointestinal disease-associated emesis.
Sedation and tranquilization for selected minor procedures in small animals.
Preanesthetic medication in selected small-animal patients.

Contraindications

Avoid use in horses because violent reactions can occur.
Use cautiously with seizure disorders or hypotension.
Hypovolemia or shock; use cautiously in cardiac disease, hepatic impairment or severe debilitation.
Do not inject IM into rabbits because of marked local injury.
Tetanus or strychnine intoxication requires an alternative plan because of conflicting source advice.

Interactions

Other sedatives increase CNS depression.
Epinephrine during alpha blockade can cause paradoxical vasodilation and tachycardia; do not use it reflexively to correct phenothiazine-associated hypotension.
Acetaminophen or dipyrone can add to hypothermia; antacids and adsorbent antidiarrheals can reduce oral absorption.
Organophosphate exposure, quinidine and propranolol require particular interaction review.

Adverse effects

Sedation
Hypotension
Antimuscarinic effects
Extrapyramidal movements

Preparations

Preparation	Form	Strength	Route
Chlorpromazine 10 mg tablet	Tablet	10 mg / tablet	PO
Chlorpromazine 25 mg tablet	Tablet	25 mg / tablet	PO
Chlorpromazine 50 mg tablet	Tablet	50 mg / tablet	PO
Chlorpromazine 100 mg tablet	Tablet	100 mg / tablet	PO
Chlorpromazine 200 mg tablet	Tablet	200 mg / tablet	PO
Chlorpromazine 25 mg/mL injection	Injection	25 mg / mL	IV, IM, SC

Monitoring and Administration

Monitor cardiovascular tolerance and sedation. Chemotherapy-associated vomiting has also been treated with 2 mg/kg subcutaneously every 3 hours in the cited material.

Protect from light. Soft PVC containers may adsorb drug; slight discoloration alone is not proof of instability.

Chlorpromazine does not provide analgesia. Monitor blood pressure, rhythm and body temperature as indicated; anesthetic requirements may fall.

ABCB1/MDR1-mutant dogs can have exaggerated prolonged sedation; a cautious initial reduction of about 25% is described. Avoid using tranquilization alone to manage aggression.

IV administration requires dilution to 1 mg/mL or less and slow delivery. Do not use markedly darkened or precipitated solution; avoid prolonged storage in plastic syringes or bags.