Phenobarbital

What is this medication for?

• Idiopathic epilepsy (first-line), status epilepticus, drowsiness
• Idiopathic epilepsy (drug of first choice), status epilepticus

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• PU/PD/polyphagia, drowsiness, wobbliness/unsteadiness (Common, dose-related)
• Ataxia, weight gain, PU/PD (Common)

Worth a phone call to your veterinarian

• Elevated liver enzymes (ALP especially) (Common; not always hepatopathy)
• Hepatotoxicity (Uncommon; more likely levels >30–40 mcg/mL)
• Neutropenia, low red blood cell counts, low platelet counts (Rare, idiosyncratic, reversible)
• Superficial necrolytic dermatitis (Rare)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Known hypersensitivity, severe liver disease, nephritis or severe respiratory depression (large doses)
• has or has had: Hypovolemia, low red blood cell counts, borderline hypoadrenal function, cardiac/respiratory disease; give into a vein slowly (≤60 mg/min), never under the skin/perivascular (irritating/necrotic); pregnancy may increase seizure frequency

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Levetiracetam
  • Zonisamide
  • Chloramphenicol, cimetidine, azoles (itraconazole)
  • Corticosteroids, cyclosporine, doxycycline, digoxin, theophylline, warfarin, beta-blockers
  • MAO inhibitors (amitraz, selegiline)

Storage and handling

Tight container, protect from light, room temperature. Alkaline solutions (pH 9–11) — do not mix with acidic solutions/flavorings; precipitation can occur.

Good to know

• Sample timing for serum levels is not time-critical relative to dose — draw any time during the interval.
• Autoinduction of hepatic metabolism means dose requirements can drift upward over time even with stable seizure control — periodic rechecks are needed even in "stable" patients.
• C-into a vein controlled substance — standard controlled-drug logging/dispensing rules apply.