Thiopental Sodium — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	IV	10-20 mg/kg (to effect)	Single dose for induction; additional small doses (1-2 mg/kg) may be given for maintenance if needed	Duration: Anesthesia lasts 5-15 minutes after a single dose Notes: Administer slowly to effect; premedication with anticholinergics and tranquilizers may reduce dose.
Cat	IV	10-20 mg/kg (to effect)	Single dose for induction	Duration: Anesthesia lasts 5-10 minutes Notes: Cats are more sensitive to respiratory depression; use lower end of dose range.
Horse	IV	8-12 mg/kg (to effect)	Single dose for induction	Duration: Anesthesia lasts 5-10 minutes Notes: Often used in combination with muscle relaxants; rapid injection may cause excitement.
Cattle	IV	10-15 mg/kg (to effect)	Single dose for induction	Duration: Anesthesia lasts 5-10 minutes Notes: Use with caution in ruminants due to bloat risk; may require endotracheal intubation.
Small Ruminants (sheep, goats)	IV	10-15 mg/kg (to effect)	Single dose for induction	Duration: Anesthesia lasts 5-10 minutes Notes: Similar to cattle; monitor for regurgitation.
Rabbit	IV	10-20 mg/kg (to effect)	Single dose for induction	Duration: Anesthesia lasts 5-10 minutes Notes: Rabbits are prone to respiratory depression; use with assisted ventilation.

Clinical Indications & Species Uses

General Indications

Induction of anesthesia
Short-term anesthesia
Anticonvulsant in emergency situations

Dog (Canine)

Induction of general anesthesia
Anesthesia for short procedures
Control of seizures (status epilepticus) in emergency settings

Cat (Feline)

Induction of general anesthesia
Anesthesia for short procedures

Horse (Equine)

Induction of general anesthesia
Anesthesia for short procedures

Cattle (Bovine)

Induction of general anesthesia
Anesthesia for short procedures

Small Ruminants (Sheep / Goat)

Induction of general anesthesia
Anesthesia for short procedures

Rabbit & Small Mammals

Induction of general anesthesia
Anesthesia for short procedures

Pharmacology & Mechanism of Action

Drug Class: Barbiturate | Pharmacological Group: General Anesthetic (Intravenous)

Mechanism of Action: Thiopental is an ultra-short-acting barbiturate that enhances the inhibitory effects of gamma-aminobutyric acid (GABA) at GABA-A receptors. It binds to the beta subunit of the receptor, increasing the duration of chloride channel opening, leading to hyperpolarization of neurons and inhibition of excitatory neurotransmission. This results in rapid induction of anesthesia and unconsciousness. At higher doses, it also suppresses excitatory glutamate receptors.

Pharmacodynamics: Thiopental produces dose-dependent central nervous system depression, ranging from sedation to deep anesthesia. It has a rapid onset of action (within 30-60 seconds) due to its high lipid solubility, which allows rapid penetration of the blood-brain barrier. It causes a decrease in cerebral metabolic rate, cerebral blood flow, and intracranial pressure. It also has anticonvulsant properties. Cardiovascular effects include hypotension due to peripheral vasodilation and myocardial depression, especially at high doses. Respiratory depression is dose-dependent, with apnea occurring at anesthetic doses.

⚡ Pharmacokinetics Summary

Absorption: Not administered orally; only IV administration. Onset is immediate after IV injection.

Distribution: Highly lipid-soluble, rapidly distributes to the brain and other highly perfused tissues. After initial effect, it redistributes to muscle and fat, leading to rapid recovery from a single dose. Volume of distribution is large.

Metabolism: Primarily hepatic metabolism via oxidation and desulfuration to inactive metabolites. Some metabolism occurs in the kidney and brain.

Excretion: Metabolites are excreted in urine, with a small fraction excreted unchanged. The elimination half-life is longer than the duration of anesthesia due to redistribution.

Half-Life: Dog: 2-4 hours; Cat: 2-3 hours; Horse: 1-2 hours; Cattle: 1-2 hours (approximate).

Bioavailability: Not applicable (IV only).

Protein Binding: Approximately 80-90% bound to plasma proteins, primarily albumin.

Available Formulations & Strengths

Injectable Solution (powder for reconstitution) 500 mg, 1 g, 2.5 g vials (reconstitute to 2.5% or 5% solution) (IV)

Contraindications & Clinical Warnings

Contraindications:

Known hypersensitivity to barbiturates
Severe hepatic disease
Severe renal disease
Respiratory obstruction or severe respiratory disease
Cardiovascular instability (e.g., shock, severe heart failure)
Porphyria (barbiturates can precipitate acute attacks)
Use in animals with a history of adverse reactions to barbiturates

Warnings & Clinical Precautions:

Administer only by slow IV injection to avoid respiratory arrest and cardiovascular collapse.
Have resuscitation equipment and drugs (e.g., oxygen, vasopressors) available.
Use with caution in debilitated, geriatric, or pediatric animals; reduce dose.
In ruminants, risk of bloat and regurgitation; consider fasting and endotracheal intubation.
May cause tissue necrosis if injected perivascularly; use a central or large vein.
Avoid extravasation; if it occurs, treat with local infiltration of lidocaine or hyaluronidase.
Use with caution in animals with hypovolemia or anemia.
May cause excitement during induction if given too rapidly or without premedication.
In horses, rapid injection may cause transient excitement or muscle tremors.
Not recommended for use in animals with head trauma unless intracranial pressure is monitored.
Use in pregnant animals only if clearly needed; may cause neonatal depression.

Adverse Effects & Reactions

Common:

Respiratory depression
Apnea
Hypotension
Tachycardia
Coughing or laryngospasm
Excitement during induction
Muscle tremors

Serious / Severe:

Cardiovascular collapse
Severe respiratory arrest
Anaphylaxis
Arrhythmias
Pulmonary edema
Cerebral edema (with rapid injection)

Rare:

Tissue necrosis at injection site
Thrombophlebitis
Hepatic necrosis
Renal failure
Blood dyscrasias

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Other CNS depressants (e.g., opioids, benzodiazepines, phenothiazines)	Additive CNS and respiratory depression; reduce thiopental dose by 25-50%.	High
Inhalant anesthetics (e.g., halothane, isoflurane)	Additive cardiovascular depression; reduce thiopental dose.	Moderate
Sulfonamides	May displace thiopental from protein binding, increasing free drug concentration and risk of toxicity.	Moderate
Hepatic enzyme inducers (e.g., phenobarbital, rifampin)	May increase metabolism of thiopental, reducing duration of action.	Mild
Hepatic enzyme inhibitors (e.g., chloramphenicol, cimetidine)	May decrease metabolism, prolonging effects.	Moderate
Neuromuscular blocking agents	Thiopental may potentiate the effects of muscle relaxants.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe respiratory depression or apnea
Hypotension
Cardiovascular collapse
Coma
Hypothermia
Pulmonary edema

Emergency Treatment Protocol: Provide supportive care: maintain airway, administer oxygen, assist ventilation (e.g., mechanical ventilation). Administer IV fluids and vasopressors (e.g., dopamine, norepinephrine) for hypotension. Monitor cardiac function. There is no specific antidote; use symptomatic and supportive treatment. Consider activated charcoal if oral ingestion (unlikely in veterinary use).

Food Animal Withdrawal Times

🥩 Meat: 1 days🥛 Milk: 1 days

Withdrawal times are not established for thiopental in food animals; use according to label or veterinary supervision. A conservative withdrawal time of at least 24 hours for meat and milk is recommended, but consult regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store powder at room temperature (15-30°C) in a dry place. Reconstituted solution should be used within 24 hours if refrigerated; discard unused portions.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light. Do not freeze. Use only if solution is clear and colorless.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for use in dogs and cats (and other species) as an injectable anesthetic; however, availability may be limited.

Extra-Label (Off-Label) Use: In the US, thiopental is not approved for use in food animals; extra-label use requires a valid veterinary-client-patient relationship and must comply with AMDUCA regulations. Withdrawal times must be extended accordingly.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Thiopental is an ultra-short-acting barbiturate used primarily for induction of anesthesia. It is characterized by rapid onset and short duration due to redistribution. It is often used in combination with other agents for balanced anesthesia. Due to its potential for severe respiratory and cardiovascular depression, it should be administered by trained personnel with appropriate monitoring and resuscitation equipment. In recent years, propofol has largely replaced thiopental in many practices due to smoother induction and recovery, but thiopental remains a cost-effective option in some settings. Always calculate dose accurately and administer to effect. Premedication with anticholinergics (e.g., atropine) may be necessary to prevent bradycardia, especially in dogs. In cats, use lower doses and monitor closely. For food animals, consider regulatory restrictions and withdrawal times.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)