A seizure is caused by abnormal electrical discharges in brain cells. In these cells, phenobarbital reduces the electrical charge, inhibiting seizure activity.
How much Phenobarbital should a dog have?
The most commonly used medication to prevent seizures is phenobarbital, also known as Epiphen. Most dogs respond well to it when given twice daily. The most common form of phenobarbital is in tablet form, but a liquid form is also available. Tablets come in a variety of sizes, so it is easy to give the right dosage to both tiny and giant dogs.
A starting dose of 2-3 milligrams (mg) per kilogram (kg) of body weight should be taken twice daily. To control seizures, most dogs require 3 mg/kg twice daily, so a 30 kg Golden Retriever would receive 90 mg twice daily. This dose may be gradually increased in some cases.
Each individual requires a different dose to control seizures. The blood concentration of drug is what is important for seizure control (see below). Different dogs break down and get rid of the drug at different rates. The concentration of phenobarbital in the blood may dip if phenobarbital is not given every 12 hours.
Phenobarbital can be given with or without food. Ideally, the dose should be taken at the same time every day to ensure that it does not get forgotten.
It is important not to abruptly stop phenobarbital therapy as the pet may experience withdrawal seizures.
Why do you need to take blood samples?
The liver breaks down phenobarbital, so long-term use may affect the liver. Regular blood tests are recommended if phenobarbital is prescribed to check for liver damage.
After 10-14 days of starting treatment, blood samples are taken to measure phenobarbital levels in the blood. It is also recommended to check the blood levels every 6-12 months, or every 2 weeks following a change in dose, as well as whenever seizure control is poor, such as following an unexpected seizure. Be aware that “trough” concentrations are not necessary. As a general rule, it is a good idea to:
1) Test the samples in the same laboratory
2) The blood samples should be obtained at the same time on each occasion
3) Before collecting a blood sample, fast the dog for at least 12 hours. It is possible for results to be affected by lipaemia (fat in the blood).
Most dogs need blood concentrations of at least 100 mol/l or 20 mg/l to control seizures. You may consider adding or changing medications if the seizures are still not adequately controlled (clusters of more than three seizures or seizures occurring more than once every six weeks) when the phenobarbital serum concentration is 25-28 mg/l (120 -140 mol/l).
What side effects can phenobarbital cause?
- Sedation and poor co-ordination – This is commonly seen at the beginning of therapy, after increases in doses, or with the addition of another drug, e.g. bromide. This effect typically wears off within two weeks. You may need to reduce the dose of phenobarbital or switch to another medication if it doesn’t or is excessive.
- Increased urination and drinking – Phenobarbital acts like a diuretic. It is imperative that the pet has access to water while on phenobarbital treatment to avoid dehydration. It is possible for some animals on high doses to wet in the house over night or for long periods of time when left alone.
- Increased appetite – Phenobarbital therapy can increase a pet’s appetite however they often do not require more food. Avoiding weight gain can be challenging. Your pet may be able to eat more if you feed a lower calorie food. The use of a Bustercube, for example, can also help extend meal times.
- Liver damage – This is a frequently talked about problem in animals on anti-epileptic medication, but in reality it is rare. During the breakdown of phenobarbital, the liver may be damaged (see factsheet on phenobarbital and the liver). The following circumstances may lead to this:
1) There is an unusual sensitivity to the drug in the animal (known as idiosyncratic reaction) – which cannot be predicted
2) Pre-existing liver disease – for this reason, blood samples are usually taken to monitor liver function before and during treatment (usually every 6-12 months).
3) Drugs are given in excessive doses over an extended period of time. Serum concentrations over 30 mg/l or 145 mol/l should be avoided.
- Pancreatitis – Epileptic dogs are at greater risk of pancreatitis – i.e. inflammation of the pancreas. In severe cases, pancreatitis can cause life-threatening symptoms such as vomiting and anorexia. Multiple risk factors are likely to contribute to this disease, such as obesity, persistently high triglyceride levels, a high fat diet, scavenging, and high phenobarbital and bromide dosages.
Blood cell abnormalities – Extremely rarely phenobarbital may induce problems with blood cell production / destruction.