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What are the side effects of phenobarbital in dogs?

Phenobarbital may cause sedation, drowsiness, ataxia, and low blood pressure in dogs
Taneia Surles, MPH
By | Apr. 10, 2025 Medically reviewed by Emma Ryan, DVM
Man and woman hugging a golden retriever: What are the side effects of phenobarbital in dogs?

Key takeaways

  • Phenobarbital is a barbiturate used in veterinary medicine to treat seizures in dogs and other small animals by stabilizing neuron activity and increasing an animal’s seizure threshold to lessen the severity and frequency of seizures.

  • Short-term side effects of phenobarbital may include sedation, agitation, drowsiness, ataxia, and increased thirst and hunger.

  • It is safe for dogs to stay on phenobarbital long-term, but regular liver testing and blood work are crucial to prevent liver toxicity, which can be life-threatening.

  • Be sure to monitor your dog for side effects, especially with long-term use of phenobarbital, and discuss lowering your pet’s dosage with their veterinarian if your dog experiences adverse side effects.

  • Dogs on phenobarbital should maintain a strict dosing schedule to sustain phenobarbital levels for proper seizure control.

In this article:

Phenobarbital (brand name Luminal) is a barbiturate used in veterinary medicine to control or prevent seizures. Also known as an anticonvulsant, phenobarbital works as a central nervous system depressant, increasing an animal’s seizure threshold to lessen the likelihood and severity of seizures. Dogs taking phenobarbital may experience mild short-term side effects or more serious long-term side effects, which is why regular blood testing is crucial to monitor liver enzyme function and blood counts.

What is phenobarbital used for in dogs?

Anticonvulsant medications are used to treat seizures in humans and canines. One of the most commonly used anticonvulsant medications (also known as antiepileptic drugs) used by veterinarians is phenobarbital, a barbiturate. Barbiturates are depressant drugs chemically derived from barbituric acid and have been approved by the FDA to control seizures in dogs.

Barbiturates work by increasing gamma-aminobutyric acid (GABA) in the brain, slowing brain cell activity, and calming and reducing seizure activity. Phenobarbital is most commonly administered to dogs orally via phenobarbital tablets or intravenously, but it can also be made into a topical gel at a compounding pharmacy and applied on the skin.

A veterinarian will prescribe phenobarbital to your dog if they are experiencing seizures or epilepsy to reduce both the frequency and severity of seizures.

The dosage of phenobarbital used depends on your dog’s condition. For severe situations, a higher dose is immediately administered. “High-dose phenobarbital is used mainly when you are loading a patient,” says Jason Horgan, DVM of LeadER Animal Specialty Hospital of South Florida. When loading a patient, a veterinarian aims to rapidly achieve therapeutic levels of the drug in the dog’s bloodstream to quickly stop the simultaneous ongoing seizure activity. Once these therapeutic levels have been achieved, lower maintenance doses on an ongoing basis become the standard to maintain proper treatment levels.

Nita Vasudevan, DVM, a veterinarian of Embrace Pet Insurance in Orlando, Florida, states that phenobarbital can also be used alongside other drugs for specialized treatment. “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,” she says. “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 provide guidance on the best dosing procedure for your pet to control seizure activity effectively.

What are the side effects of phenobarbital in dogs?

While it is crucial to get your pet’s seizures under control, phenobarbital can have a variety of short-term side effects for dogs. According to veterinary experts, common side effects of phenobarbital may include:

  • Sedation 
  • Agitation
  • Excessive drowsiness
  • Wobbling gate (ataxia)
  • Increased thirst
  • Increased hunger
  • Increased urination

Many of these side effects occur due to the initial high dosage of phenobarbital and can be reduced by prescribing a lower dose. Pets that have transitioned to the maintenance dose stage may see side effects reduce or disappear completely. Pet owners concerned about their pet’s side effects can also talk to their veterinarian about trying a different anticonvulsant medication to control their seizures.

Phenobarbital is generally well-tolerated by most dogs, and it is common for side effects to lessen or even go away completely as a dog’s body gets used to the drug. You may notice that your dog is sleepier or clumsier than usual, as the drug targets the brain and nervous system, affecting their coordination and energy. However, this is not something to be concerned about unless your dog’s symptoms become severe.

With proper dosing, phenobarbital is safe for your dog to take and can help keep their seizures under control.

How long can a dog stay on phenobarbital?

Your dog can safely stay on phenobarbital long-term with the right treatment plan. Dogs with epilepsy or chronic seizures typically require medication to keep their condition under control, and phenobarbital is often the first-line medication many veterinarians prescribe.

The most important thing for dogs taking phenobarbital long-term is to undergo regular blood and liver testing to ensure that their 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.”

Dogs taking phenobarbital long-term are at risk for developing liver toxicity, which can lead to liver failure without an adjustment to the medication dosage. Dr. Horgan also notes that phenobarbital is a liver enzyme inducer. This means that exposure to the drug can make a dog’s liver more efficient at metabolizing it, which can lower the blood levels of the drug and lead to recurring seizures, which is why regularly testing blood levels is so important.

This regular testing is the safest way to know if your dog is taking the correct dosage of phenobarbital or if the dosage or dosing schedule needs to be adjusted for better results.

Long-term side effects of phenobarbital in dogs

It’s important to monitor your dog long-term, as side effects may occur in pets taking phenobarbital for extended periods and can be life-threatening.

Veterinarian experts note that possible long-term side effects of phenobarbital to watch out for in dogs may include:

  • Decreased liver function
  • Liver damage
  • Sedation
  • Weight gain
  • Reduced blood cell counts (in rare cases)

If you start to notice any concerning side effects in your dog with long-term use of phenobarbital, please contact your vet immediately to discuss the possibility of lowering your dog’s dosage or trying a different medication.

Tips for dogs on phenobarbital

With proper dosage and care, phenobarbital can greatly improve the quality of life of epileptic dogs. While the drug does come with possible side effects, there are ways to mitigate adverse effects and reduce seizures.

Below are some tips to support your pet during their phenobarbital treatment:

  • Get regular testing: Regular testing is key to ensure that dogs are receiving the proper dose of phenobarbital for their needs. “The goal is to always find the lowest, most effective dose to be as safe as possible,” Dr. Vasudevan says. Discovering the sweet spot of a dosage that is high enough to prevent seizures but low enough to prevent side effects can be tricky, but it is crucial to ensure a dog’s overall health.
  • Try alternatives: Levetiracetam (Keppra), potassium bromide, and zonisamide are a few popular seizure control medications that can be used individually or in combination to minimize seizures for dogs.
  • Be aware of drug interaction: Certain antihistamines, antibiotics, and supplements may interact negatively with phenobarbital, so pet owners should consult their veterinarian to discuss any medications their dog is already taking. Additionally, dogs with liver disease should not take phenobarbital.
  • Determine dosing schedule: Pet owners should consult with their veterinarian to determine the best dosing schedule for their dog. Phenobarbital is generally taken two to three times per day, either with or without food.
  • Maintain a strict dosing schedule: Pet owners should avoid missing doses as much as possible to ensure the medication maintains daily therapeutic levels for adequate seizure control.
  • Get liver supplements: Dogs whose liver is affected by phenobarbital may take liver support medications, such as Denamarin.

The most important thing to remember is to maintain good communication with your veterinarian to ensure that your pet lives the best possible life.