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Phenobarbital dose for dogs and cats, steered by the serum level

Phenobarbital starts at 2.5 to 3 mg/kg PO every 12 hours in dogs and 1 to 2 mg/kg PO every 12 hours in cats, and the Vetinuity phenobarbital dose table below then moves the dose by the serum phenobarbital level, aiming for roughly 15 to 45 µg/mL measured about 2 weeks after any change.

By Vetinuity, founded by Brian Toncray, DVM, emergency veterinarian · October 26, 2026

Phenobarbital doses at a glance

Vetinuity phenobarbital dose calculator

Where the dose comes from

The 2015 ACVIM consensus on seizure management in dogs rates phenobarbital as a first-line maintenance drug and recommends the twice-daily oral starting dose in the table¹. The International Veterinary Epilepsy Task Force gives the same starting range and builds every later adjustment on the measured serum concentration rather than on the seizure count alone². In cats, a once-daily oral schedule has also been reported to control presumed idiopathic epilepsy, which helps owners who cannot medicate twice a day³.

Reading the serum phenobarbital level

Phenobarbital takes about two weeks to reach steady state, so the first level is drawn roughly 14 days after starting and again 14 days after every dose change². Most dogs are controlled somewhere between 15 and 45 µg/mL; the task force suggests aiming toward the middle of that span and treating the upper end as the zone where liver injury becomes more likely^2,1. A dog that is still seizing with a level low in the range gets a proportional dose increase: new dose equals current dose multiplied by the desired level divided by the measured level. A dog seizing with a level near the top needs a second drug, not more phenobarbital¹.

Liver values on phenobarbital

Phenobarbital induces liver enzymes, so a raised ALP, and often a raised ALT, is expected in a treated dog and does not on its own mean hepatotoxicity; in one study enzyme activity matched poorly with what the biopsy showed⁴. The values that change the dosing decision are the ones that measure liver function: falling albumin, rising bilirubin, and abnormal pre- and post-prandial bile acids. When those appear, or when the serum level sits above the upper target, the dose comes down and a second drug takes over the load².

Blood counts, especially in cats

Neutropenia, thrombocytopenia and anemia are uncommon but recognized on phenobarbital. In a retrospective review of 69 treated cats, hematologic changes appeared in a minority and resolved in several after the drug was stopped, so a CBC belongs on every recheck panel in the first months⁵. A cat with a new cytopenia on phenobarbital is switched to another antiseizure drug rather than given a lower dose.

Related values on this site: ALT, ALP and bilirubin, Platelet count, PCV and hematocrit.