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Phenobarbitone for Dogs: The Ultimate Guide

Did you know phenobarbital is the most commonly used drug to treat canine idiopathic epilepsy? It's a powerful medication that can help control seizures in dogs. In this article, we will discuss why phenobarbital is used to treat canine epilepsy, and we'll also cover the side effects of the drug. So if your dog has been diagnosed with epilepsy, read on for more information about this important medication.

Phenobarbital MOA

Phenobarbital is a barbiturate medication that is used to treat canine epilepsy. It works by depressing the central nervous system and helps control seizures in dogs. Its main anticonvulsant effects are as a GABA receptor agonist. However, it also can be considered a broad-spectrum anticonvulsant, as it inhibits glutamate and calcium fluxes.

Whats GABA?

GABA is a neurotransmitter that plays a crucial role in epilepsy. It's responsible for sending messages between nerve cells and helps regulate the nervous system. When GABA levels are low, seizures can occur. Phenobarbitone helps to increase GABA levels in the brain, which helps to prevent seizures.

What are the side effects of phenobarbital?

Overall, phenobarbital is well tolerated in companion animals; however, like all medications, there are some potential mild and more severe side effects. When first commencing treatment, it is common for patients to experience 1-2 weeks of lethargy, sedation and ataxia (wobbliness). Usually, these side effects are transient as the body adjusts to the medication. Long-term side effects include increased appetite (polyphagia), increased thirst (polydipsia) and increased urination (polyuria). Weight gain is a common sequel to increased appetite.

Does phenobarbital affect liver enzymes?

An elevation in liver enzymes is commonly seen with phenobarbital use; however, an elevation alone does not directly correlate with hepatic damage. Phenobarbital concentrations > 35 mcg/ml are suspected to be associated more frequently with liver damage than benign enzyme activation. Additional testing, such as measuring the therapeutic index, bile acids and albumin monitoring, can be helpful in determining if an elevation of hepatocellular enzymes (liver enzymes) on a routine blood test is significant.

How often should phenobarbitone levels be checked?

It takes two weeks for serum phenobarbital levels to reach a steady state after commencing medication or a dose change. This means, a routine blood test is taken just before the medication is given (i.e. at the drug trough) every two weeks after starting or any alterations to dosage are made. In patients on a stable phenobarbital dose, serum levels monitoring is usually carried out every 3 to 6 months.

Why do we monitor phenobarbital levels?

Phenobarbital levels are monitored to ensure the patient has an appropriate dose. Ideally, we want the patient to be having the lowest effective dose.

Why would phenobarbital levels decrease?

With prolonged treatment, it is not uncommon to begin to see a decrease in the therapeutic level. There are usually two main reasons why (1) an increase in patient body weight and (2) an increase in clearance secondary enhanced metabolism.

Does phenobarbitone affect other drugs?

Yes, phenobarbital can affect the metabolism of other drugs. This is due to its inducing effect on the enzyme CYP450, which enhances clearance of other medications that are metabolism by the liver. Commonly used drugs that are affected by phenobarbital include; doxycycline, praziquantel, itraconazole, corticosteroids, theophylline, cyclosporine, doxorubicin and metronidazole.

Moreover, taking phenobarbital can make diagnosing some other metabolic diseases significantly more challenging. For example, its use interferes with the testing for hyperadrenocorticism (Cushing's disease) and hypothyroidism (low thyroid).

Why would my dog be changed off phenobarbital onto another seizure medication?

There are a few reasons your vet may change your dog to another medication. The most common reason is inadequate seizure control on phenobarbital alone or the development of significant side effects that cannot be resolved. Although rare, side effects such as liver scarring or anaemia would prompt a change to another anti-epileptic medication.


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