Phenobarbital is an antiepileptic drug that belongs to the group of barbiturates. Due to its high efficacy, it is considered the drug of first choice to treat seizures caused by epilepsy in dogs. However, its administration can cause the appearance of various adverse reactions, which is why it is necessary to periodically monitor the concentrations of the drug in the blood to always maintain the levels within the therapeutic range.
If you want to know more about phenobarbital in dogs, what it is used forand the recommended dose , don't hesitate to read the following article on our site where we talk about the dose and what this drug is for.
What is phenobarbital?
Phenobarbital is an antiepileptic drug belonging to the barbiturate family. It is the oldest antiepileptic used in veterinary medicine and is considered the drug of first choice for the treatment of epilepsy in dogs due to its high availability and high effectiveness.
Currently available in tablet form for oral administration in dogs.
What is phenobarbital used for in dogs?
Phenobarbital in dogs is used to prevent seizures due to epilepsy As a clarification, we will say that epilepsy consists of an alteration of the brain function characterized by the periodic, recurrent, and unpredictable occurrence of seizures. These seizures occur due to the disorderly and rhythmic firing of cortical neurons, leading to synchronous electrical discharges. For more information, do not miss this other article on Epilepsy in dogs, causes, symptoms and treatment.
All anticonvulsant drugs, including phenobarbital, base their mechanism of action on stabilizing the neuronal membrane to prevent his excitement. That is, they suppress excess neuronal electrical activity to prevent the onset of seizures. Specifically, the 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.
It should be noted that although anticonvulsants are used to treat epilepsy, they are only symptomatic treatments. This means that does not cure epilepsy, but only lessens its symptoms (seizures).
When to start administering phenobarbital in dogs?
Despite the fact that, in veterinary medicine, there is no consensus on when antiepileptic treatment should be started, most studies in this regard recommend starting treatment when any of the following criteria is met:
- When 2 seizures occur in a 6-month period.
- 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 seizures occur in a row, which is commonly known as “ cluster seizures”. This criterion should be considered even if seizures are 2-3 hours apart.
- When interictal periods are shortened, that is, the periods between one seizure and another.
In the event of a severe crisis, in this article we explain how to deal with an epileptic seizure in dogs.
Dog Phenobarbital Dosage
The initial dose of phenobarbital should be 2-5 mg per kg of body weight per day. This dose should be divided and given twice a day.
Due to the side effects that we will see later, it is important to measure the 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 the blood should range between 15 and 40 µg/ml, with the optimum being around 30 µg/ml.
After starting a 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 drug levels to equilibrate in the blood.
- Epilepsy is not controlled: If the seizures continue, your veterinarian will measure the levels of phenobarbital in blood to decide what decision to make. If levels are low, the dose of phenobarbital may be increased. If the levels are within the normal range, phenobarbital may be combined with other anticonvulsants, such as potassium bromide or imepitoin.
- The epilepsy was controlled, but at some point the convulsive crises reappear: Similarly, in this case the levels of phenobarbital to decide whether to increase the dose or to combine it with other anticonvulsant drugs.
Phenobarbital overdose in dogs
Phenobarbital overdose in dogs usually occurs as a result of accidental ingestion of the drug. This is a serious intoxication in which the following symptoms: may occur
- Central nervous system depression, which can range from drowsiness to coma.
- Respiratory problems.
- Cardiovascular problems, hypotension and anaphylactic shock, which can lead to kidney failure and death of the animal.
Given the seriousness of phenobarbital overdose, it is essential that as soon as possible intoxication is detected or suspected, you go to a veterinary center as an emergency to avoid drug absorption and to provide respiratory and cardiovascular support when needed. Although there is no specific antidote for this intoxication, central nervous system stimulants, such as Doxapram, can help stimulate the respiratory center.
In any case, remember the importance of keeping any medicinal product away from your pets, since it will be the most effective way to avoid poisoning by accidental ingestion.
Side Effects of Phenobarbital for Dogs
Most side effects associated with phenobarbital appear at the beginning of treatment or after an increase in dose, and usually improve or disappear after 1-2 weeks. Next, we collect the main adverse effects of phenobarbital in dogs:
- Polyuria: increased urine volume.
- Polydipsia: increased water consumption.
- Polyphagia: increased food intake.
- Sedation and ataxia.
- Behavioral changes: such as paradoxical hyperexcitability.
- Hepatotoxicity: in prolonged treatments, liver parameters (liver enzymes and bile acids) should be monitored.
- Cytopenias: reduction in the number of blood cells.
- Reduction in T4 or thyroxine levels: in any case, the levels return to normal 4-6 weeks after stopping the treatment with phenobarbital.
Much less frequently, other adverse effects may occur such as superficial necrolytic dermatitis, pancreatitis and dyskinesias (abnormal and involuntary movements).
Contraindications of phenobarbital for dogs
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 in dogs are:
- Liver failure.
- Anemia.
- Serious renal or cardiovascular disorders.
- Allergy to the active ingredient, to any other barbiturate or to any of the drug's excipients.
In addition, although its use is not expressly contraindicated during pregnancy and lactation, it should be used with caution in these physiological situations:
- Phenobarbital is capable of crossing the placental barrier, which can affect the growth of puppies and favor neonatal hemorrhage. However, if the veterinarian considers that the benefits of the treatment 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.
