Phenobarbital For Dogs - Uses, Dosage, Uses and Side Effects

Phenobarbital is an antiepileptic drug belonging to the barbiturate family. It is the oldest antiepileptic used in veterinary medicine and is the drug most commonly used to treat epilepsy in dogs. It is widely available and highly effectiveness. For this reason, it is on the World Health Organization's list of essential medicines. It is currently available in tablet form for oral administration in dogs.

Phenobarbital in dogs is used to prevent epileptic seizures. Epilepsy is a broad term for a range of disorders which are characterized by seizures in dogs. These seizures occur due to the disorderly and rhythmic firing of cortical neurons, leading to synchronous electrical discharges.

All anticonvulsant drugs base their mechanism of action on stabilizing the neuronal membrane to prevent their excitation. This includes phenobarbital for dogs. They suppress excess neuronal electrical activity to prevent the onset of seizures. The specific anticonvulsant effect of phenobarbital is due to the fact that it enhances the action of the neurotransmitter GABA, the main inhibitory neurotransmitter of the central nervous system (CNS).

Although anticonvulsants are used to treat epilepsy, they are only symptomatic treatments. This means that they do not cure epilepsy, but only lessen or manage its symptoms, i.e. seizures.

Administering phenobarbital to dogs

In veterinary medicine, there is no consensus on when antiepileptic treatment should be started. Despite this fact, most studies related to phenobarbital for dogs recommend starting treatment when any of the following criteria is met:

• When 2 seizures occur within a period of 6 months.
• If there are severe postictal signs (signs that follow the seizure) such as aggressiveness or blindness. Also, when these postictal signs have a duration equal to or greater than 24 hours.
• When multiple seizures occur, commonly known as ‘cluster seizures in dogs’. This criterion should be considered even if seizures are 2-3 hours apart.
• When the interictal periods are shortened, i.e. the periods between one convulsive seizure and another.

To know what to expect before, during and after these episodes, take a look at our article on why my dog is having seizures.

As with any medication prescribed by our veterinarian, it is vital we follow their instructions closely. Generally speaking, the initial dose of phenobarbital should be 2-5 mg per kg of body weight per day. This dose should be divided in two and given in two separate administrations every 12 hours.

It is important to measure the blood serum levels of the drug to adjust the dose and keep it within the therapeutic range. As serum concentrations of phenobarbital do not reach a steady state until 2 weeks after the start of treatment, the dose should not be adjusted during this period of time. Thereafter, serum levels should be monitored every 6 months. Normally, plasma concentrations of phenobarbital in blood should range between 15 and 40 µg/ml, with the optimum being around 30 µg/ml.

After starting treatment with phenobarbital, the following situations may occur:

• The epilepsy is controlled but the patient is heavily sedated: in these cases, your veterinarian may reduce the dose of phenobarbital or wait a few days for the drug levels to equilibrate in the blood.
  
  
• Epilepsy is not controlled: if the seizures continue, your veterinarian will measure the levels of phenobarbital in the blood to decide on the next course of action. If levels are low, the dose of phenobarbital may be increased. If the levels are within the normal range, phenobarbital can be combined with other anticonvulsants, such as potassium bromide or imepitoin.
  
  
• The epilepsy was controlled, but seizures subsequently reoccur: in this case, the levels of phenobarbital will be measured to decide whether to increase the dose or if it is combined with other anticonvulsant drugs.

Epilepsy can occur in dogs of any age, but it can be related to neurological issues which affect dogs as they age. Learn more with our article on neurological problems in older dogs.

Phenobarbital overdose in dogs usually occurs as a result of accidental ingestion of the drug. It is a serious intoxication in which the following symptoms can occur:

• Central nervous system depression: can range from drowsiness to coma.
• Respiratory problems: can cause the dog to stop breathing in severe intoxication.
• Cardiovascular problems: this can include hypotension.
• Anaphylactic shock: can lead to kidney failure and death of the animal.

Given the seriousness of phenobarbital overdose, it is essential the dog is taken to an emergency veterinary clinic as soon as intoxication is detected or even suspected. Immediate treatment may be the only way to provide respiratory and cardiovascular support. Although there is no specific antidote for this poisoning, central nervous system stimulants, such as doxapram, can help stimulate the respiratory rate.

Always keep any medication out of reach of your pet, whether it is for veterinary or human use. This will be the most effective way to avoid poisoning due to accidental ingestion.

Most side effects associated with phenobarbital appear at the beginning of treatment or after an increase in the dose. They usually improve or disappear after 1-2 weeks. Below you will find the main adverse effects of phenobarbital in dogs:

• Polyuria: increased volume of urine.
• Polydipsia: increased water consumption.
• Polyphagia: increased food intake.
• Sedation: dogs may appear lethargic and sluggish.
• Ataxia: lack of muscle coordination.
• Behavioral changes: such as paradoxical hyperexcitability.
• Hepatotoxicity: in prolonged treatments, liver parameters (liver enzymes and bile acids) should be monitored.
• Cytopenia: reduction in the number of mature blood cells.
• Reduction of T4 or thyroxine levels: the levels return to normal 4-6 weeks after stopping treatment with phenobarbital.

Other adverse effects such as superficial necrolytic dermatitis, pancreatitis and dyskinesias (abnormal and involuntary movements) may appear, although they are much less frequent. Persistent or severe side effects need to be seen by a veterinarian.

Despite being a very effective drug, there are certain situations in which the administration of phenobarbital may not be recommended. The main contraindications of phenobarbital occur in dogs with:

• Liver failure
• Anemia
• Severe renal or cardiovascular disorders
• Allergy to the active ingredient or any excipient of the drug

Although its use is not expressly contraindicated during pregnancy and lactation, it should be used with caution in these physiological situations:

• Phenobarbital is able to cross the placental barrier, being able to affect the growth of puppies and favor neonatal hemorrhages. If the veterinarian considers the benefits of the treatment to outweigh the risks, the drug may be prescribed to pregnant bitches.
• Phenobarbital is excreted in small amounts in breast milk and may cause sedative effects in nursing puppies. In these cases, you can opt for early weaning and bottle-feeding the litter.

Learn more about other neurological disorder symptoms in dogs with this related article on why a dog is fainting all the time.

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

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