Major Uses

Phenobarbital is a barbiturate drug with anticonvulsant properties that is often used to reduce the severity and frequency of seizures in dogs and cats with epilepsy. It may also be used at higher doses to produce sedation.

Phenobarbital is a controlled substance schedule IV and can only be obtained through a DEA-licensed veterinarian. It is not approved by the Food and Drug Administration for use in animals, but it is prescribed legally as an extra-label drug for this purpose.

Common Precautions

Side effects associated with use of phenobarbital include increased thirst, increased appetite, restlessness, lethargy, depression, and anxiety. Some side effects may fade with continued use of the medication. When taken at higher doses, phenobarbital is a sedative that causes nervous system depression.

Prolonged use of phenobarbital can cause significant liver impairment. The benefits of long-term use should be weighed against this risk.

Phenobarbital should be used with caution in animals with liver, heart, or lung disease and in those with anemia. If anemia develops while an animal is undergoing treatment with phenobarbital, the medication should be stopped and a new anti-seizure drug should be selected.

This drug may interact with other supplements and medications, including narcotics, antihistamines, corticosteroids, certain antibiotics, griseofulvin, beta-blockers, Lysodren, quinidine, theophylline, and tranquilizers.

Phenobarbital should not be used in animals with a known allergy or hypersensitivity to the drug. Overdose of phenobarbital can cause significant sedation or even death. Any known or suspected overdose should be reported to a veterinarian right away.


Phenobarbital should be stored in its original packaging, at room temperature, and out of the reach of children and animals.


Phenobarbital is available in 1/4 grain, 1/2 grain, 1 grain, and 100 mg tablets. Grains are an old unit of measurement, and one grain is equal to 60 mg. Phenobarbital is also available as an elixir and for injection.

Generally, phenobarbital dosing in dogs begins at 1 to 2 mg/lb (2 to 4 mg/kg) twice daily. In cats, the typical starting dose is 7.5 to 15 mg per cat twice daily. If necessary, the dose may be increased.

Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely.

If a dose of phenobarbital is missed, it should be taken as soon as possible. If it is almost time for the next dose, the missed dose should be taken immediately and the medication schedule should be adjusted. Missing even a single dose of phenobarbital can trigger a seizure. It is important not to take two doses of this medication at the same time.

This information is for general reference only and should not be used for the diagnosis or treatment of any condition of your pet. It's intended as a general reference, this information may not include all possible uses, precautions, directions, reactions (including allergic), drug interactions, or withdrawal times. Always consult your local veterinarian and have your pet examined for any advice concerning the diagnosis and treatment of your pet, including which products and doses are most appropriate. Any trademarks are the property of their respective owners. VetDepot is not a pharmacy. All prescription products are dispensed by our Pharmacy Partner. Article last updated 2/2014.