Phenobarbital
Phenobarbital is a barbiturate anticonvulsant.
Group
Barbiturate anticonvulsant
Mechanism of Action
Enhances GABA-mediated chloride conductance and stabilizes neuronal activity.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Epilepsy | 2–8 mg/kg | PO | Every 12 h | |
| Status epilepticus | 10–20 mg/kg | IV | As needed | Give increments to effect; monitor cumulative exposure and respiration. |
| Epilepsy — Initial | 2–2.5 mg/kg | PO | Every 12 h | Titrate to seizure control and serum concentration. |
| Epilepsy — Initial | 2.5–3 mg/kg | PO | Every 12 h | Titrate to seizure control and serum concentration. |
| Epilepsy — Initial | 3.5 mg/kg | PO | Every 12 h | Titrate to seizure control and serum concentration. |
| Rapid anticonvulsant loading — Loading | 16–20 mg/kg | IV | Once | |
| Epilepsy — Maintenance | 2–5 mg/kg | PO | Every 12 h | |
| Status epilepticus | 5–8 mg/kg | IV | Every 4–6 h | After benzodiazepines; reassess respiration and cumulative exposure. |
| Sialadenosis case report | 1 mg/kg | PO | Every 12 h | Single case after esophageal foreign-body removal; tapered after 3 months, not established universal therapy. |
| Sedation | 2.2–6.6 mg/kg | PO | Every 12 h | |
| Irritable bowel syndrome adjunct | 2.2 mg/kg | PO | Every 12 h |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Epilepsy — Initial | 1–2 mg/kg | PO | Every 12 h | Increase gradually to tolerated maintenance dose. |
| Epilepsy — Maintenance | 2–4 mg/kg | PO | Every 12 h | |
| Status epilepticus | 10–20 mg/kg | IV | As needed | Give increments to effect; monitor cumulative exposure and respiration. |
| Epilepsy with transdermal formulation | 9 mg/kg | TOPICAL | Every 12 h | PLO gel or Lipoderm; adjust using serum concentrations because exposure is variable. |
| Rapid anticonvulsant loading — Loading | 16–20 mg/kg | IV | Once | |
| Epilepsy — Maintenance | 1–5 mg/kg | PO | Every 12 h | |
| Situational vocalization or distress | 2–3 mg/kg | PO | As needed |
Horses
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Epilepsy | 12 mg/kg | PO | Every 24 h | |
| Epilepsy | 12 mg/kg | PO | Every 12 h | Every-12-hour alternative may be needed after initial treatment. |
| Seizure control | 5–20 mg/kg | IV | Infuse over 30 minutes; sodium chloride may be used for dilution. | |
| Foal seizure control | 2–5 mg/kg | IV | Foals only: give over 20 minutes, starting at the low end. | |
| Seizure control — Loading | 12 mg/kg | IV | Once | Over 20 minutes. |
| Seizure control — Maintenance | 6.65 mg/kg | IV | Every 12 h | Over 20 minutes. |
| Epilepsy — Loading | 16–20 mg/kg | IV | Once | |
| Epilepsy — Maintenance | 1–5 mg/kg | PO | Every 12 h | Adult horse regimen. |
| Foal seizure control — Loading | 20 mg/kg | IV | Once | Dilute to 30-35 mL saline and infuse over 25-30 minutes. |
| Foal seizure control — Maintenance | 9 mg/kg | IV | Every 8 h | Foals; dilute and infuse as for loading. |
| Foal oral seizure maintenance | 100–500 mg/animal | PO | Every 12 h | Follows 16-20 mg/kg IV loading; individualize by serum level. |
Ferrets
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Rapid anticonvulsant loading — Loading | 16–20 mg/kg | IV | Once | |
| Epilepsy — Maintenance | 1–2 mg/kg | PO | Every 8–12 h | |
| Epilepsy | 1–2 mg/kg | PO | Every 8–12 h |
Indications
- Epilepsy
- Status epilepticus
- Foal seizures
- Canine sialadenosis
- Rapid anticonvulsant loading
- Sedation
- Irritable bowel syndrome adjunct
- Situational vocalization or distress
- Seizure control
- Foal seizure control
- Foal oral seizure maintenance
Contraindications
- Hepatic disease requires caution.
Interactions
- Potent enzyme induction can lower many concurrent drug concentrations, including levetiracetam, theophylline, corticosteroids and cardiovascular drugs.
- Avoid mixing alkaline injection with acidic medicines.
Adverse effects
- Sedation, ataxia, polyphagia, hepatopathy; idiosyncratic cytopenias, pancreatitis with bromide and superficial necrolytic dermatitis.
Toxicity
Overdose causes profound sedation, ataxia, hypothermia and coma. Support ventilation/circulation; repeated activated charcoal can enhance elimination when airway protection permits. Dialysis or other enhanced-elimination treatment is reserved for severe cases.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Phenobarbital 15 mg | Tablet | 15 mg / tablet | PO |
| Phenobarbital 30 mg | Tablet | 30 mg / tablet | PO |
| Phenobarbital 60 mg | Tablet | 60 mg / tablet | PO |
| Phenobarbital 100 mg | Tablet | 100 mg / tablet | PO |
| Phenobarbital oral elixir 4 mg/mL | Oral Liquid | 4 mg / mL | PO |
| Phenobarbital Lipoderm 250 mg/mL | Other | 250 mg / mL | TOPICAL |
| Phenobarbital sodium injection 30 mg/mL | Injection | 30 mg / mL | IV |
| Phenobarbital sodium injection 60 mg/mL | Injection | 60 mg / mL | IV |
| Phenobarbital sodium injection 65 mg/mL | Injection | 65 mg / mL | IV |
| Phenobarbital sodium injection 130 mg/mL | Injection | 130 mg / mL | IV |
Monitoring and Administration
Adjust to seizure control and serum concentrations. Reported therapeutic ranges: dogs 15-40 mcg/mL, or 10-36 with bromide; cats have reported ranges of 15-45 with narrower study targets; horses 15-20. Monitor CBC, liver function and lipids.
Sampling time is relatively unimportant; stored separator tubes may lower measured concentrations. Total thyroid hormone results can be altered.
An oral 10 mg/mL suspension made from tablets in equal Ora Plus/Ora Sweet was stable for 115 days in amber plastic at room temperature. Lipoderm 250 mg/mL retained potency for 8 weeks; feline absorption is variable.
Emergency dog/cat protocol uses 2-5 mg/kg boluses after diazepam, repeated every 20 minutes up to twice, with a separate 2-10 mg/hour phenobarbital infusion. Feline alternative uses 3 mg/kg IV every 20 minutes to cumulative 24 mg/kg in 24 hours, or a 10 mg/kg loading bolus. Cumulative weight-based limits are not per-dose caps.
Newer cited concentration targets emphasize lower exposure: dogs approximately 20-35 mcg/mL, cats 23-30 mcg/mL. Obtain CBC, chemistry and urine baseline; measure serum after 2-3 weeks and dose changes. Stable patients need periodic levels and hepatic/CBC monitoring; enzyme induction can increase clearance. A route-unspecified compulsive-behavior range and a cyclic bovine 5-g enzyme-induction report lack complete schedules and are not calculator protocols.