Phenobarbital: veterinary dose and monograph

At a glance

Drug class

Sedative / anaesthetic

Also known as

Phenobarbitone, Luminal

Species with doses

Dogs, Cats, Horses and Ferrets

General dose range

2–5 mg/kg PO q12h — check the species tables for the indication

Routes

PO, IV

Last reviewed

1 October 2026

Indications

• Idiopathic epilepsy maintenance — dogs and cats
• Status epilepticus / cluster seizures (after benzodiazepine)
• Sedation / anxiolysis (occasional)
• Adjunctive therapy for compulsive behaviours
• Excessive feline vocalisation during car travel

Phenobarbital dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Idiopathic epilepsy — initial maintenance: Plumb’s p.960: start 2–2.5 mg/kg PO q12h (Munana); 2.5–3 mg/kg q12h (Mariani); 3.5 mg/kg q12h (Axlund). Papich 5e: 2–8 mg/kg q12h PO. Check trough serum concentration 2–3 weeks after starting or any change (target 20–35 µg/mL) and monitor liver function as levels approach 30–35 µg/mL; some dogs auto-induce and need higher doses.
• Status epilepticus — IV loading: Plumb’s p.960: loading 16–20 mg/kg IV once (Knipe); 20 mg/kg IV to reach therapeutic levels quickly (Podell); after benzodiazepines 5–8 mg/kg IV every 4–6 h until controlled (Knipe) or 2–5 mg/kg boluses repeated at 20-min intervals up to twice (Quesnel). Papich 5e: 10–20 mg/kg IV in increments to effect. Give slowly; expect sedation and respiratory depression.
• Sedation / compulsive behaviour adjunct: Plumb’s p.960: sedation 2.2–6.6 mg/kg PO twice daily (Walton); irritable bowel 2.2 mg/kg twice daily; compulsive behaviours 2–20 mg/kg q12–24h (Line).

Cats

• Idiopathic epilepsy — initial maintenance: Plumb’s p.960: 1–2 mg/kg (usually 3.25–15 mg per cat) PO q12h (Shell; Cochrane); 2–2.5 mg/kg q12h (Munana); maintenance after a load 1–5 mg/kg q12h (Knipe). Papich 5e: 2–4 mg/kg q12h, starting at 1–2 mg/kg; transdermal 9 mg/kg q12h. Target trough 23–30 µg/mL.
• Status epilepticus: Plumb’s p.960 (Abramson 2009): with IV diazepam, phenobarbital 3 mg/kg IV repeated every 20 min up to 24 mg/kg in 24 h, or a 10 mg/kg IV loading bolus; loading 16–20 mg/kg IV (Knipe). Papich 5e: 10–20 mg/kg IV in increments to effect.

Horses

• Foals — seizure control: Papich 5e: foals 2–5 mg/kg IV slowly over 20 minutes, starting low. Plumb’s p.961 (Spehar 1984): foals 20 mg/kg diluted and infused over 25–30 min IV, then 9 mg/kg q8h; or load 16–20 mg/kg IV then 100–500 mg (total) PO q12h (Knipe 2009). Monitor serum levels.
• Adult — IV loading for seizures: Plumb’s p.961: 12 mg/kg IV over 20 min then 6.65 mg/kg IV over 20 min q12h (Duran 1987); or 16–20 mg/kg IV once (Knipe 2009). Papich 5e: 5–20 mg/kg IV over 30 minutes (may dilute in saline).
• Adult — oral maintenance: Papich 5e: 12 mg/kg q24h PO; some horses need 12 mg/kg q12h after initial therapy. Plumb’s (Knipe 2009) lists 1–5 mg/kg PO q12h after an IV load. Adjust by serum concentration.

Ferrets

• Seizure control: Plumb's p.961 (Williams 2000): 1–2 mg/kg PO 2–3 times daily. Status: loading 16–20 mg/kg IV once, then 1–2 mg/kg PO q8–12h (Knipe 2006/2009).

Contraindications

Severe liver disease, nephritis, marked respiratory depression (large doses), known hypersensitivity. Use cautiously in cats — particularly sensitive to barbiturate respiratory depression. Caution: hypovolaemia, anaemia, borderline hypoadrenalism, cardiac/respiratory disease. Many drug interactions — induces metabolism of corticosteroids, cyclosporine, doxycycline (effect persists weeks after discontinuation), levothyroxine, theophylline, digitoxin, beta-blockers, oral anticoagulants. Never combine with carprofen — increased hepatotoxicity risk.

Species-specific warnings

Dogs: First-line for idiopathic epilepsy. Target trough 25–30 µg/mL. Monitor liver q6mo. Don't stop abruptly — withdrawal seizures.

Cats: Cats more prone to phenobarbital hepatotoxicity. Levetiracetam often safer. KBr CONTRAINDICATED in cats (asthma).

Horses: Used for foal neonatal seizures and adult refractory epilepsy. 5–10 mg/kg IV/PO load, then 2–5 mg/kg q12h. Strict drug-testing regulations.

Adverse effects

• Dogs: transient sedation, anxiety/agitation or lethargy when initiating therapy
• Dogs: PU/PD/polyphagia at moderate-high serum levels (mimics Cushing's)
• Dogs: ataxia, sedation at higher trough levels
• Dogs: elevated ALT/ALP — hepatotoxicity uncommon but real (esp. trough >35 µg/mL)
• Cats: ataxia, persistent sedation, polyphagia/weight gain
• Rare: anaemia, thrombocytopenia, neutropenia (immune-mediated, reversible)
• Rare in dogs: superficial necrolytic dermatitis (SND)
• Coagulopathy in cats at very high doses (10–40 mg/kg/day)

Drug interactions

• CNS depressants (opioids, benzodiazepines, alpha-2 agonists, anaesthetics): additive sedation
• Hepatic enzyme inducer: ACCELERATES metabolism of MANY drugs — corticosteroids, theophylline, doxycycline, metronidazole, beta-blockers, calcium channel blockers, warfarin, cyclosporine, NSAIDs (partial)
• Chloramphenicol, cimetidine, fluoroquinolones (especially marbofloxacin): INHIBIT phenobarbital metabolism — toxicity risk
• Valproate, felbamate: increase phenobarbital levels
• Phenytoin: complex interaction — may increase or decrease either drug
• KBr (potassium bromide): commonly co-administered for refractory epilepsy — synergistic
• Acepromazine: lowers seizure threshold — AVOID in epileptic dogs

Pregnancy and lactation

Crosses placenta. Teratogenic in humans (oral clefts, congenital heart defects) — extrapolated risk in animals. Neonatal withdrawal can occur. Continue in well-controlled epileptic bitches as seizures pose greater risk than the drug; in mild epilepsy, may withdraw if clinically safe. Enters milk in clinically relevant amounts — monitor neonates for sedation.

Monitoring

• Serum phenobarbital trough at 14 days post-start, then every 6 months on stable dose; target trough 20–35 µg/mL (dogs), 15–30 µg/mL (cats)
• Hepatic enzymes (ALT, ALP, bile acids) at baseline, 30 days, then every 6 months
• CBC every 6 months — rare blood dyscrasia
• PU/PD/polyphagia documentation — expected but excess = consider Cushing's workup
• T4 — phenobarbital decreases total T4 (be aware when investigating thyroid status)
• Seizure log — frequency, duration, post-ictal — adjust dose to clinical control

Toxicity and overdose

Acute overdose: profound sedation, ataxia, respiratory depression, hypothermia, hypotension. Chronic: hepatotoxicity (idiosyncratic + cumulative), PU/PD/polyphagia, sedation, ataxia. Cats: more prone to hepatotoxicity than dogs — KBr toxicity (asthma-like syndrome) makes alternatives like levetiracetam preferred. Bone marrow suppression rare. Trough serum >35 µg/mL = increasing risk of hepatotoxicity.

Management: No specific antidote. Supportive intensive care: airway protection, mechanical ventilation if respiratory depression, IV fluids for hypotension, active warming for hypothermia, urinary alkalinisation with sodium bicarbonate (urine pH 7.5–8) accelerates excretion. Activated charcoal q4–6h × 24 h (enterohepatic recirculation). Hemodialysis in severe cases. Discontinue concurrent CYP inhibitors.

Clinical pearls

• Dogs idiopathic epilepsy: 2–3 mg/kg PO q12h starting dose, titrate to trough 25–30 µg/mL over weeks
• Status epilepticus: 4–16 mg/kg IV loading over 30 min (split into 2-mg/kg boluses every 20–30 min until effect), then maintenance PO
• Levetiracetam (Keppra) is replacing phenobarbital as first-line for many vets — no hepatotoxicity, easy titration; phenobarbital still has higher seizure-free rates
• KBr add-on (35 mg/kg PO q24h) for breakthrough seizures on max phenobarbital
• Cats: 1–2 mg/kg PO q12h — start low. Hepatotoxicity more common than in dogs
• NEVER acepromazine in epileptic dogs — lowers threshold
• Take 14 days for steady state — do NOT adjust dose before then unless emergency
• Owner education: never stop suddenly (withdrawal seizures); monitor for jaundice, lethargy, anorexia (hepatotoxicity)

Mechanism of action

Long-acting barbiturate anticonvulsant. Binds the barbiturate site on GABA-A receptors, prolonging chloride channel opening and increasing neuronal inhibition. Raises seizure threshold and depresses cortical excitation. Strong hepatic enzyme inducer (cytochrome P450, especially CYP2B11 / CYP3A4) — induces its own metabolism (auto-induction) and that of many co-administered drugs. Half-life dogs ~40–90 h; cats ~30–50 h; takes 10–14 days to reach steady state.

Formulations and products

• Tablet 15 mg
• Tablet 30 mg
• Tablet 60 mg
• Elixir 4 mg/mL
• Injection 65 mg/mL
• Injection 130 mg/mL

Storage

Tablets / elixir: 15–30 °C, dry. Schedule IV controlled substance (US). Injection: 15–25 °C, protect from light.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other sedatives and anaesthetics

• Ketamine HCl
• Medetomidine
• Midazolam
• Pentobarbital (Pentobarbitone)
• Propofol
• Romifidine
• Sevoflurane
• Thiopental Sodium

Frequently asked questions