What is Phenobarbital?

Cat on Phenobarbital being checked by vet

Phenobarbital (Epiphen, Soliphen, Phenoleptil) is a highly effective anti-epileptic drug which is widely-used in dogs. It has been used since the sixties. Extensive research has been done that ensures safety and efficacy.

Phenobarbital can be administered by mouth (tablets) or by injection. The latter are used in emergency situations in a veterinary hospital setting. Once administered, it reaches the highest blood concentration after around 4 hours. It is metabolised by the liver. Phenobarbital crosses the so-called ‘brain-blood barrier’, meaning it is active in the brain environment. The drug molecules bind to specific neuro-receptors in the brain and this binding inhibits the excessive electrical activity in the brain that leads to seizure activity.

Phenobarbital helps in controlling epilepsy by reducing the frequency, duration and severity of seizures (up to 85% in one study) and preventing the formation of new epileptic foci, which otherwise leads to progression of the disease. We consider the drug is effective if the seizure frequency reduces by half.

Why has my pet been prescribed phenobarbital tablets?

Phenobarbital is usually the ‘first line medication’ of choice in dogs with seizures for various causes, including idiopathic epilepsy. It is also used in dogs with seizures due to meningitis, head trauma or brain tumours.

How frequently will my pet need to take phenobarbital?

Generally, phenobarbital is given every 12 hours. Timing is crucial as delays in medication can cause withdrawal seizures. As far as possible, the medication should be given at the same times every day. If a dose is delayed, give it as soon as possible and continue with the next tablet at the usual time.

Phenobarbital can be given with food. It can be given generally with other anti-epileptic medications but check with your veterinary clinician about this before proceeding.

What is the dose of phenobarbital my pet will need?

Your veterinary surgeon will advise you on the initial dose of phenobarbital required to treat your pet; the dose will depend on the liver metabolism of your dog, and doses vary greatly from dog to dog. It is likely that the dose will also vary over time, so your may dog may end up with a dose much higher than the one initially prescribed. Always follow your vet’s advice when changing doses. The drug concentration in the blood needs to be carefully monitored and maintained at the right level to ensure good control of the seizures. Phenobarbital usually takes 14 to 20 days to be effective, so your pet may continue to have seizures during this period.

Will my pet require blood tests during treatment?

Phenobarbital concentration in your pet’s blood must be checked regularly to ensure it is high enough to be effective but not high enough to incur a high risk phenobarbital toxicity. We advise checking phenobarbital concentration, liver function and full blood cell count by blood sampling 3 weeks after starting medication or modification of the dose and then every 3 to 6 months for life. Usually, these blood tests can be done at your local practice.

What are the side effects of phenobarbital?

The most common side effects of phenobarbital are increased thirst (polydipsia) and urination (polyuria), increased hunger, weight gain and sedation. These side effects are common, and usually improve greatly after the first 14 to 20 days; some dogs may experience hyperexcitability or behavioural changes.

Severe side effects affecting the liver or the blood cell count are rare, but dangerous: blood tests will be performed periodically to ensure it is safe for your pet to continue phenobarbital treatment. If your pet is unwell, has no appetite or he appears under the weather, you should check with your veterinarian immediately.

Are there alternative treatments to phenobarbital?

Alternatives to phenobarbital can be considered if your dog has a history of liver problems, if the response to treatment is inadequate or if the side effects of the drug are intolerable. Your veterinarian or neurologist will discuss alternative treatments with you.

If my pet is taking other medication, could phenobarbital have an adverse effect?

Generally, phenobarbital can be taken along with your pet’s other medications. However, phenobarbital may reduce the effect of some drugs or exacerbate the effects of others. It usually does not interfere with worming treatment or flea treatments, so you can continue these as usual. For other medications, it is best to check with your veterinarian or neurologist.

Can I continue to vaccinate my pet if phenobarbital has been prescribed?

Most vaccines are safe to give and we advise you to continue with your regular vaccination plan while your dog is on phenobarbital.

My dog has been on phenobarbital for more than 3 weeks, but the seizures have not reduced. What should I do?

The first step would be to check your dog’s phenobarbital serum levels, biochemistry and blood cell count. If all is fine, the next step would be to increase the dose of phenobarbital. Your vet will advise on the new dose to give. If this measure is not enough, a “second line” anti-epileptic drug will be added. Most dogs will respond to phenobarbital or the combination of phenobarbital and a second line anti-epileptic drug.

Remember to keep all medications out of the reach of children and use gloves when handling tablets. If you have doubts please always check with a veterinary surgeon.

Arranging a referral for your pet

If you would like to refer your pet to see one of our Specialists please visit our Arranging a Referral page.