Key takeaways
Phenobarbital is a barbiturate used in veterinary medicine to prevent and control seizures in dogs.
Short-term side effects of phenobarbital in dogs may include sedation, agitation, drowsiness, wobbly gait, and increased thirst and hunger.
Dogs can safely stay on phenobarbital long-term, but regular liver testing and blood work are crucial to prevent life-threatening liver toxicity.
Phenobarbital is a long-acting barbiturate (a class of depressant drugs) used in veterinary medicine to control or prevent seizures in dogs. It works by increasing the seizure threshold in the brain, reducing the likelihood and severity of seizures. Dogs taking phenobarbital may experience mild short-term side effects, and more serious side effects are possible with long-term use, which is why regular blood testing is important to monitor liver function and blood counts.
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What is phenobarbital used for in dogs?
Anticonvulsant medications (also called antiepileptic drugs) are commonly used to treat seizures and epilepsy in dogs. Phenobarbital is one of the most widely prescribed, and Fidoquel CA-1, a brand of phenobarbital labeled for dogs, has been conditionally approved by the FDA to control seizures in dogs.
Barbiturates work by increasing gamma-aminobutyric acid (GABA), a neurotransmitter that slows nerve activity and reduces seizures. Phenobarbital is most commonly given orally as tablets or intravenously, but it can also be compounded into a topical gel applied to the skin.
How is phenobarbital dosed in dogs?
The phenobarbital dosage depends on the dog’s condition. For severe cases, a higher dose is given immediately. “High-dose phenobarbital is used mainly when you are loading a patient,” says Jason Horgan, DVM, of LeadER Animal Specialty Hospital in South Florida. Loading refers to rapidly building up therapeutic drug levels in the bloodstream to stop ongoing seizure activity quickly. Once those levels are reached, lower maintenance doses become the standard.
Phenobarbital can also be used alongside other medications. “Low doses of this medication are used for the management of pets who have rare or inconsistent seizures, [and sometimes a low dose is used] along with a combination of anti-seizure medications to obtain adequate seizure control,” says Nita Vasudevan, DVM, a veterinarian at Embrace Pet Insurance in Orlando, Florida. “High doses are used in pets who have more severe or cluster seizures, or some pets who are still refractory, even with a combination of seizure medications.”
Your veterinarian can advise on the best dosing approach for your dog.
What are the side effects of phenobarbital in dogs?
Phenobarbital can cause a variety of short-term side effects in dogs. Common side effects of phenobarbital in dogs include:
- Sedation
- Agitation
- Excessive drowsiness
- Wobbling gait (ataxia, or loss of coordination)
- Increased thirst
- Increased hunger
- Increased urination
Many of these side effects are linked to the initial high loading dose and may reduce or disappear once a dog transitions to a lower maintenance dose. If side effects persist, a veterinarian may consider adjusting the dosage or switching to a different anticonvulsant medication to better manage your dog’s seizures.
You may notice your dog is sleepier or clumsier than usual. This is generally not a cause for concern unless symptoms become severe. Phenobarbital is well-tolerated by most dogs and, with proper dosing, can effectively keep seizures under control.
Long-term side effects of phenobarbital in dogs
Dogs on phenobarbital long-term may experience side effects that, in some cases, can be life-threatening.
Possible long-term side effects to watch for include:
- Profound Sedation
- Lack of Appetite
- Frequent vomiting
- Yellow coloring of the skin or gums (jaundice)
- Weakness
Possible long-term adverse effects of phenobarbital include:
- Decreased liver function
- Liver damage
- Reduced blood cell counts (in rare cases)
If you notice any concerning side effects, contact your veterinarian right away to discuss adjusting the dosage or trying a different medication.
How long can a dog stay on phenobarbital?
Dogs with epilepsy or chronic seizures typically require long-term medication, and phenobarbital is often the first-line treatment veterinarians prescribe. With the right treatment plan, dogs can safely stay on phenobarbital long-term.
The most important thing for dogs on phenobarbital long-term is regular blood testing to adjust the dosage, monitor for serious side effects, and ensure the body is metabolizing the medication well.
“Testing is crucial for this medication, typically every 6-12 months,” Dr. Vasudevan says. “Typically, as veterinarians, we will also want to test the liver enzyme functionality along with phenobarbital. It is possible that blood work has to be done more frequently initially while we are figuring out regulation for the pet.”
Liver toxicity and enzyme induction
Dogs on phenobarbital long-term are at risk for liver toxicity, which can lead to liver failure if the dosage isn’t adjusted. Dr. Horgan notes that phenobarbital is a liver enzyme inducer, meaning the drug causes the liver to become more efficient at breaking it down over time. As a result, blood levels of the drug can drop, potentially leading to recurring seizures. This is why regular blood level monitoring is so important.
In rare cases, phenobarbital can also cause low blood cell counts, so blood counts are included in routine testing.
Tips for dogs on phenobarbital
With proper dosage and care, phenobarbital can greatly improve the quality of life for dogs with epilepsy. Here are some ways to support your dog during treatment and minimize side effects.
- Get regular testing: Regular testing is key to ensuring dogs receive the right dose of phenobarbital. “The goal is to always find the lowest, most effective dose to be as safe as possible,” Dr. Vasudevan says. Finding a dosage high enough to prevent seizures but low enough to avoid side effects can be tricky, but it is crucial to your dog’s overall health.
- Try alternatives: Levetiracetam (Keppra), potassium bromide, and zonisamide are seizure control medications that can be used individually or in combination with phenobarbital.
- Be aware of drug interactions: Certain antihistamines, antibiotics, and supplements may interact negatively with phenobarbital. Always tell your veterinarian about any medications your dog is already taking. Dogs with liver disease should not take phenobarbital.
- Follow a consistent dosing schedule: Phenobarbital is generally taken two to three times per day, with or without food. Avoid missing doses to maintain therapeutic drug levels for adequate seizure control.
- Consider liver supplements: Dogs whose liver is affected by phenobarbital may benefit from liver support medications such as Denamarin, a supplement commonly recommended by veterinarians to support liver health.
Maintaining open communication with your veterinarian is the best way to ensure your dog stays healthy and their treatment remains on track.
- Anticonvulsants for the treatment of animals, Merck Veterinary Manual (2025)
- Barbiturates, Cleveland Clinic (2022)
- FDA conditionally approves phenobarbital tablets to control seizures in dogs with idiopathic epilepsy, U.S. Food & Drug Administration (2023)
- Phenobarbital, Clinician’s Brief (2015)
- Phenobarbital, Veterinary Partner (2024)
- Plumb’s Veterinary Drug Handbook, 7th ed. (2011)
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