Barbiturates
Barbituric acid derivatives
Also known as: Phenobarbital · Pentobarbital · Thiopental · Methohexital · Barbituric acid derivatives
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Dosing by species
Dosing data for this drug is not yet available in the VetSheet formulary. Every other section below is complete. Consult the current product label before prescribing.
Overview
Barbiturates are a class of drugs acting as central nervous system (CNS) depressants. They can invoke all levels of CNS mood alteration, ranging from paradoxical excitement to deep coma and death.
Key systemic effects include:
- Respiratory: Dose-dependent respiratory depression. At sedative doses, depression mimics normal sleep. As doses increase, the medullary respiratory center is progressively depressed.
- Cardiovascular: Can cause tachycardia, decreased myocardial contractility, decreased stroke volume, and decreased mean arterial pressure.
- Gastrointestinal: Generally reduces tone and motility of intestinal musculature. Thiobarbiturates may cause a transient increase in tone/motility after initial depression.
- Musculoskeletal: Reduces sensitivity of the motor end-plate to acetylcholine, providing mild skeletal muscle relaxation.
- Hepatic: Chronic administration (especially phenobarbital) leads to well-documented hepatic microsomal enzyme induction.
Clinical Pearl: Barbiturates possess no true intrinsic analgesic activity. They are primarily utilized in veterinary medicine for seizure management (e.g., phenobarbital), anesthesia induction (e.g., thiopental, methohexital), and humane euthanasia (e.g., pentobarbital).
Mechanism of action
The exact mechanisms for the CNS effects caused by barbiturates are complex and multifactorial:
- Neurotransmitter Inhibition: They inhibit the release of excitatory neurotransmitters including acetylcholine, norepinephrine, and glutamate.
- GABA Modulation: Barbiturates enhance inhibitory neurotransmission by acting on GABA receptors. Pentobarbital is specifically noted to be GABA-mimetic, directly activating GABA receptors at higher doses.
- Calcium Channel Blockade: At high anesthetic doses, barbiturates inhibit the uptake of calcium at nerve endings, further dampening synaptic transmission.
Safety & warnings
Contraindications
- Use with extreme caution in cats due to high sensitivity to respiratory depressant effects
Adverse effects
- Dose-dependent respiratory depression
- Cardiovascular depression (hypotension, decreased contractility)
- Paradoxical excitement
- Hypothermia (at anesthetic doses)
- Decreased gastrointestinal motility
Precautions
Species Sensitivity: Must be used very cautiously in cats, as they are particularly sensitive to the respiratory depressant effects of barbiturates.
Organ Function: Severe renal impairment may occur secondary to hypotensive effects in overdose situations. Extended use induces hepatic microsomal enzymes, which can alter the metabolism of concurrently administered medications.
Respiratory/Cardiac Arrest: Respiratory arrest may occur at doses four times lower than those that cause cardiac arrest.
Monitoring
- Respiratory rate, depth, and volume
- Heart rate and blood pressure
- Body temperature (monitor for hypothermia)
- Liver enzymes (with chronic administration)
Pharmacokinetics
Metabolism
Hepatic metabolism. Extended administration induces hepatic microsomal enzymes.
Overdose
Overdose can lead to profound CNS depression, coma, and death.
- Respiratory arrest typically precedes cardiac arrest (occurring at doses approximately four times lower than those causing cardiac arrest).
- Severe renal impairment may occur secondary to profound hypotensive effects during an overdose.
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Client information
Barbiturates are a family of medications used in veterinary medicine for various reasons, including controlling seizures, inducing anesthesia for surgery, or for humane euthanasia.
- Monitoring: If your pet is prescribed a barbiturate for long-term use (such as phenobarbital for epilepsy), regular blood tests will be required to monitor liver function and ensure the drug levels in the blood are safe and effective.
- Side Effects: You may notice lethargy, increased thirst, or increased appetite when your pet first starts taking these medications.
- Safety: These are strictly controlled prescription medications. Keep them securely stored away from children and other pets. Never change the dose without consulting your veterinarian.
Important: If your pet appears excessively sleepy, has difficulty breathing, or is uncoordinated, contact your veterinarian immediately.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.