Phenobarbital for Dogs: Monitoring, Side Effects and CBD
Phenobarbital has been part of veterinary seizure care for longer than any other anti-seizure medicine. If your veterinarian has prescribed phenobarbital for dogs with epilepsy, the routine around the tablet matters as much as the tablet itself. That routine means scheduled blood tests, a watch on side effects and a complete list of everything else your dog takes.
That list is where CBD comes in. Owners ask about it often, and researchers have studied it in dogs that were already taking phenobarbital. Those studies are worth understanding before the conversation with your veterinarian, not in place of it.
Key takeaways
- Keep the blood-level and liver-health checks your veterinarian schedules, and bring a seizure log to each recheck.
- Do not stop phenobarbital suddenly or change the amount yourself. Follow your veterinarian’s treatment and emergency plans.
- CBD studies used specific formulations alongside existing anti-seizure medicines. They do not establish a replacement for prescriptions or safety for every dog.
What Phenobarbital Is
Phenobarbital is an anti-seizure medicine. The International Veterinary Epilepsy Task Force notes that it has the longest history of long-term use of any anti-seizure drug in veterinary medicine.
For decades, veterinarians in the United States prescribed phenobarbital tablets made for the human market. In September 2023, FDA conditionally approved a phenobarbital tablet for seizures associated with idiopathic epilepsy in dogs. It was the only phenobarbital tablet to hold that status when the agency announced it.
The liver breaks down phenobarbital, which is one reason your veterinarian reviews liver health, other medicines and follow-up bloodwork when prescribing it.
The Blood Tests Phenobarbital for Dogs Needs
Monitoring is part of the prescription, not an extra. There are two kinds of tests: one measures the drug, and the others look at the dog.
The Phenobarbital Level
The University of Illinois veterinary neurology service describes checking a phenobarbital level about two to three weeks after starting treatment or changing the amount, and about every six months thereafter. The blood level is interpreted with the seizure history and side effects, not on its own.
Your veterinarian may adjust this schedule for your dog. Ask when the sample should be collected relative to a tablet, and follow the clinic’s instructions.
Liver Values and Blood Counts
The International Veterinary Epilepsy Task Force guidance describes baseline blood counts, biochemistry and bile-acid testing, with follow-up monitoring at three months and then six-month intervals. Your veterinarian chooses the appropriate tests and timing for your dog.
A raised liver enzyme is not the same measurement as liver function. Ask how changes will be interpreted and whether additional tests are needed. Tell any clinician ordering other blood tests that your dog takes phenobarbital.
Side Effects Veterinarians Watch For
Early effects can include sleepiness, unsteady walking, increased hunger, thirst and urination. Some lessen over the following weeks; report severe, persistent or unexpected changes promptly.
Less common reactions can affect blood-cell counts or the skin. Your veterinarian will interpret symptoms with bloodwork rather than assuming every change is an expected side effect.
Do not stop phenobarbital suddenly or change the amount yourself. Abrupt withdrawal can provoke seizures. Any change needs a veterinary plan.
How Phenobarbital Interacts With Other Medicines
Phenobarbital increases the activity of some liver enzymes and can change how other medicines are cleared. Your veterinarian needs a complete list of prescriptions, parasite preventives, over-the-counter products, supplements and CBD.
Include rescue seizure medicine in the discussion. Ask for a written plan covering when to use it and when to seek emergency care; do not adjust it from information in an article.
What the Published CBD Studies Measured
Most of the clinical research on CBD in dogs with epilepsy comes from the neurology group at Colorado State University. In both clinical trials below, CBD was added to the anti-seizure medicines the dogs already took. None of these trials tested CBD in place of a prescription.
The Earlier Controlled Trial
A 2019 randomized trial enrolled 26 dogs with seizures despite medication; 16 were included in the final analysis. Two dogs receiving CBD were withdrawn after developing ataxia, or unsteady walking.
The CBD group had a median 33% reduction in seizure frequency. However, the proportion of dogs reaching at least a 50% reduction did not differ significantly between CBD and placebo. ALP, a liver enzyme, increased in the CBD group. The trial studied CBD alongside existing anti-seizure treatment, not as its replacement.
The Larger Crossover Trial
A 2023 crossover trial enrolled 51 dogs. After a lower-dose group of 12 showed limited benefit, 39 dogs formed the higher-dose analysis. The study compared three-month CBD and placebo periods while dogs continued their anti-seizure medicines.
Seizure days fell by 24.1% during CBD treatment and rose by 5.81% during placebo. The number of dogs reaching at least a 50% reduction did not differ significantly. Liver enzyme changes, decreased appetite and vomiting were also reported.
Phenobarbital levels were about 11% higher during CBD treatment, but the difference was not statistically significant. Owner-reported seizures, possible unblinding through blood results and sample size limit interpretation. A change in one study measure does not establish a result for an individual dog or another CBD product.
Interaction Research
A 2021 study in nine healthy beagles found no statistically significant pharmacokinetic interaction between the tested CBD and phenobarbital regimens. It also reported gastrointestinal effects in five dogs, hypoxia in five and increased ALP in four. Healthy research dogs do not establish safety for dogs with epilepsy taking medicines long term.
A laboratory study using canine liver tissue found faster CBD metabolism in samples from phenobarbital-treated dogs. This was not a trial in living dogs and cannot by itself establish a clinical interaction.
Why CBD Is a Question for Your Veterinarian or Neurologist
FDA has not approved CBD for use in animals. If your dog takes phenobarbital, whether CBD belongs anywhere in the picture is a decision for the veterinarian who reads your dog’s bloodwork.
The reasons are specific. Phenobarbital and CBD can both raise liver enzyme values, which makes the liver panel harder to read. Phenobarbital changes how the liver breaks CBD down, at least in the laboratory. And hemp products differ in what they contain besides CBD, including whether a trace of THC is present.
For a dog whose seizures continue despite medication, your veterinarian may involve a board-certified veterinary neurologist. That is the right setting for the CBD question. If your dog already gets CBD, tell your veterinarian, and bring the bottle and its batch lab report. Do not add, stop or change anything, phenobarbital included, without your veterinarian.
Questions to Ask Your Veterinarian
- When is my dog’s first phenobarbital level due, and how often will you check it after that?
- Which blood tests are you running at each recheck, and is a bile acid stimulation test part of the plan?
- If my dog’s liver enzymes rise, how will you tell an expected lab change from a sign of liver damage?
- Which side effects should fade in the first weeks, and which ones should make me call you?
- Which of my dog’s other medicines, flea and tick products or supplements could interact with phenobarbital?
- What is our written emergency plan if a seizure lasts more than five minutes or several come close together?
- My dog takes CBD, or I have been considering it. Should it be part of the plan at all, and would you want extra bloodwork if it were?
- Should we see a veterinary neurologist, and what records should I bring to that appointment?
Bring a seizure log to each visit: the date, the time, how long each seizure lasted and what your dog did before and after. With that log, the blood results and a complete list of what your dog takes, your veterinarian has what is needed to adjust the plan.
These products have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease.
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