Phenobarbital

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Barbiturate (Systemic Drug)

Prescriber Highlights

  • Used primarily as an antiseizure medication
  • Also used as a sedative
  • CNS depressant, but the exact mechanism of action is unknown
  • Can also invoke paradoxical excitement 
  • Class IV controlled substance

Uses, Indications

  • First-line treatment for epilepsy in dogs and cats
  • Used to prevent recurrence of seizures in status epilepticus following the use of a benzodiazepine to halt initial seizure activity
  • Also used to prevent excessive drooling (ie, sialadenosis) in dogs

Contraindications, Precautions, Warnings

  • Contraindications

Contraindicated in patients with severe liver disease or hypersensitivity to barbiturates 

  • Precautions
    • When administering IV, give slowly to avoid respiratory depression
    • Use with caution in patients that are hypovolemic or anemic or have borderline hypoadrenal function or cardiac or respiratory disease
  • Warnings

Do not give commercially available injectable preparations subcutaneously or perivascularly because significant tissue irritation and necrosis can occur

Side Effects

  • Polyuria, polydipsia, polyphagia, sedation, and ataxia are most common
  • Elevated liver enzymes (ALP, ALT) that are not necessarily indicative of liver dysfunction
  • Anemia, thrombocytopenia, leukopenia, superficial necrolytic dermatitis, and hepatic failure are seen rarely
  • Facial pruritus, immune-mediated reactions, and bone marrow hypoplasia  may also be seen in cats

Drug Interactions

  • Multiple drugs can increase the effect of phenobarbital when used concurrently
  • Can interact with an extensive list of drugs, causing a decrease in efficacy of those drugs

Laboratory Considerations

  • Alterations in thyroid testing and dexamethasone suppression tests

Monitoring

  • Anticonvulsant (or sedative) efficacy
  • Side effects
  • Serum phenobarbital levels
    • After initiation, check phenobarbital serum levels within ≈12 to 14 days in dogs and ≈9 to 10 days in cats to confirm therapeutic levels
    • Following the initial recheck, check levels every 6 months or if side effects or reduced efficacy is observed
  • If used chronically, perform serial monitoring of CBCs, liver enzymes, and bilirubin at least every 6 months 

Client Information

  • May be given with or without food 
  • Give at approximately the same time every day; do not skip any doses
  • Sedation, greater thirst and appetite, and frequent urination are common when starting therapy

Dosage Forms

  • No veterinary-labeled products are available 
  • Human-labeled products available for extra-label use include: 
    • Phenobarbital oral tablets: 15 mg, 16.2 mg, 30 mg, 60 mg, 90 mg, 100 mg
    • Phenobarbital injectable solution: 65 mg/mL, 130 mg/mL

Compiled and summarized from Plumb’s Veterinary Drugs by Shannon Palermo, VMD

Information about this drug was adapted from Plumb’s Veterinary Drugs. Further details and more therapeutics can be found with a subscription at plumbsveterinarydrugs.com.

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