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Your dog’s been prescribed phenobarbital. They may be taking it for a long time—even the rest of their life. So, it’s natural to have questions.
Phenobarbital is an anti-seizure drug. These drugs are usually prescribed for dogs that have had a history of two or more seizures, cluster seizures (two or more within 24 hours), or prolonged seizures (lasting more than 5 minutes).
What Is Phenobarbital Used for in Dogs?
Phenobarbital (commonly known as Luminal® or Barbita®) is a barbiturate that’s often the first choice for dogs with seizures. It’s affordable, easy to give, and generally both effective and safe. It has a very long history as an anti-seizure drug, being first used in humans for that purpose in 1912. It works by depressing the central nervous system, thus elevating the threshold for neurons to fire.
“Seizures result from hyperexcitable and/or hypersynchronous brain activity,” explains epilepsy researcher Karen Munana, DVM, DACVIM (Neurology), of the North Carolina State University College of Veterinary Medicine. “Phenobarbital exerts its effect by increasing inhibition of brain activity, through action on the gamma-aminobutyric acid (GABA) receptors. GABA is one of the main inhibitory neurotransmitters in the nervous system.”
Phenobarbital’s generally effective in reducing seizures whether they’re caused by epilepsy, brain tumors, poisoning, or infection. It has a good success rate, with studies showing 82% of treated dogs have at least a 50% reduction rate of seizures. However, it may not work for every dog.
Several other anti-seizure drugs, such as potassium bromide, primidone, and levetiracetam, may be given in place of or along with phenobarbital. A veterinarian or veterinary neurologist should make the decision of what drug to use, or whether any treatment should be started, as it is not a one-size-fits-all choice.
“The benzodiazepines (diazepam and midazolam) also act on the GABA receptors,” says Munana. “Other anti-seizure medications exert their effect in other ways. For example, levetiracetam (Keppra®) acts by binding to a specific protein found in the neuron terminal that decreases the release of neurotransmitters.”
How to Administer Phenobarbital
Phenobarbital is often given as a tablet, but also comes in capsule, liquid, paste, or chewable forms. It can also be given as an injectable. It’s usually given two or sometimes three times a day, with or without food.
Giving this drug on a regular schedule is very important. The drug’s effects are short-lived, so missing just one dose can allow a seizure. In the event you do miss a dose, don’t double up on it. Just give the regular dose as soon as you remember, unless it’s close to the next dosing time.
Phenobarbital is absorbed fairly rapidly, with maximal concentration in the blood reached within 4 to 8 hours of taking it by mouth. It takes 10 to 14 days to reach its “steady state,” so a much higher dose, consisting of up to four doses of higher levels of medication given over a 2 to 24-hour period (usually by injection), may be given at the start of therapy. Its effects occur much faster when given by injection.
Because individual dogs metabolize phenobarbital to different extents, some dogs may need more or less of the drug compared to others. This is why drug monitoring, especially at the beginning of treatment, is crucial.
Store the pills away from light, at room temperature (not refrigerated).
Side Effects of Phenobarbital
Like most anti-seizure medications, phenobarbital can have side effects. Sedation is common for the first week or two that the dog is taking phenobarbital, but most dogs will get over this. If not, tell your veterinarian, as sedation may indicate the dose should be lowered. If your vet does lower the dose, they may prescribe a second anti-seizure medication, such as potassium bromide.
Many dogs have increased hunger and thirst when they’re on phenobarbital. As a result, you’ll need to make sure your dog doesn’t overeat while on the medication. You may also have to take steps to prevent your dog from soiling the house if they drink too much.
Dogs on phenobarbital should have regular blood tests to check for liver toxicity, which can be a problem from prolonged high levels of the drug. Catching this early can prevent liver disease. Liver monitoring should begin two weeks after the first dose is given, then six weeks after that first dose, and thereafter every six months, according to the American College of Internal Medicine protocol.
Meanwhile, watch for signs of liver problems, such as vomiting, loss of appetite, jaundice, or small skin sores, especially on the paws. Call your veterinarian immediately if you notice any of these symptoms.
When Might Phenobarbital Not Be the Best Choice?
Because of the possibility of liver toxicity, phenobarbital may not be the best choice for dogs that already have liver disease. It may also be contraindicated in dogs with kidney inflammation or infection or dogs with severe breathing problems. It should be used with caution in dogs that are anemic, dehydrated, have heart or lung disease, poor adrenal function, or are pregnant or nursing.
It may also be difficult to use phenobarbital in dogs with hypothyroidism or Cushing’s disease, as it interferes with test results for adrenal and thyroid gland function. It also decreases the effectiveness of Lysodren, a drug used to treat Cushing’s disease).
Interactions with Other Drugs
Be sure to tell your veterinarian any other drugs your dog is taking, as phenobarbital can interact with many of them. Some medications can increase the risk of liver toxicity when given at the same time as phenobarbital. Some of these drugs include:
- Rifampin, an antibiotic
- Acetaminophen, a pain reliever
- Rimadyl (carprofen), an NSAID pain reliever
- Selegiline, a cognitive dysfunction medication
- Amitraz, an anti-tick or mite medication
In addition, the antibiotic rifampin can reduce the effectiveness of phenobarbital when given together. Medications that can enhance the action of phenobarbital when given together include any sedative, any antihistamine that can cause drowsiness, and the antibiotic chloramphenicol.
Phenobarbital will also decrease the effectiveness of some drugs including:
- Chloramphenicol and doxycycline (antibiotics)
- Corticosteroids
- Levetiracetam
- Certain cardiac drugs, including beta blockers and quinidine (a heart rhythm drug)
- Theophylline (airway dilator)
- Itraconazole (antifungal medication)
- Doxorubicin (chemotherapy drug)
- Metronidazole (anti-diarrhea medication)
- Praziquantel (dewormer)
- Cyclosporine (immunosuppressant)
When Can Dogs Discontinue Using Phenobarbital?
Sometimes dogs will be seizure-free, and the decision is made to wean a dog off anti-seizure medications. This must be done gradually, as doing so suddenly can increase the chance of a seizure. Never discontinue the use of an anti-seizure medication without first consulting with your veterinarian.
This article is intended solely as general guidance, and does not constitute health or other professional advice. Individual situations and applicable laws vary by jurisdiction, and you are encouraged to obtain appropriate advice from qualified professionals in the applicable jurisdictions. We make no representations or warranties concerning any course of action taken by any person following or otherwise using the information offered or provided in this article, including any such information associated with and provided in connection with third-party products, and we will not be liable for any direct, indirect, consequential, special, exemplary or other damages that may result, including but not limited to economic loss, injury, illness or death.