Side Effects of Phenobarbital for Dogs

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Phenobarbital is a popular and widely used medication for dogs that is typically administered for the treatment of seizures in dogs. In the case of dogs with epilepsy, phenobarbital will often be the first medication your vet will prescribe.  

Usually phenobarbital is administered as tablets, but sometimes liquid phenobarbital is used as well. Phenobarbital tablets can range in size, and will be prescribed based on your dog’s weight.

It appears to be most effective when given to a dog twice a day, with as close to 12 hours in between doses as possible. It is important to be consistent and to not miss a dose, because missing a dose could trigger the very seizures the drug is intended to treat.

Phenobarbital consumption must be closely monitored by your vet so that your dog’s blood levels remain within normal limits. On one hand, if phenobarbital levels increase too much, it can be toxic, and on the other hand, if the levels are too low, it will not do its job.

What is Phenobarbital?

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Phenobarbital is classified as a barbiturate and a nonselective CNS (central nervous system) depressant. It is otherwise known as an anticonvulsant drug. Often, phenobarbital is prescribed alone and is usually effective in 60 to 80 percent of dogs that are diagnosed with idiopathic epilepsy.

However, if phenobarbital on its own does not appear to be getting results, it may sometimes be used in conjunction with other anticonvulsant medications to achieve the desired result.

How Does Phenobarbital Work?

Phenobarbital works by decreasing and stabilizing the neuron activity in a dog’s brain. It does this by increasing the activity in a dog’s GABA neurotransmitter, and reducing the activity in a dog glutamate neurotransmitter.

The GABA neurotransmitter possesses nerve calming properties, and the glutamate neurotransmitter possesses nerve stimulating properties. When GABA activity is stimulated and glutamate activity is suppressed, this can help to reduce the number of seizures a dog may be experiencing.

What is Phenobarbital Used For?

Phenobarbital is primarily used to treat seizures in dogs (and humans), especially in dogs that have been diagnosed with epilepsy. Dogs diagnosed with epilepsy suffer from seizures on a regular basis, sometimes experiencing several seizures in one day. Phenobarbital helps to control those seizures and reduce their frequency.

However, phenobarbital does come with quite a few unpleasant potential side effects. With the advances in medicine, it is possible there may be other treatments better suited for your dog that could be prescribed in its place.

Unfortunately, a lot of the newer medications tend to be more expensive, so phenobarbital continues to be the drug of choice due to its effectiveness and cheaper price tag.

When Should Phenobarbital Not Be Used?

Phenobarbital should not be given to dogs with any of the following health concerns:

If you do choose to use this drug in dogs with any of these conditions, it should be very closely supervised by your vet. Certain drugs should not be used with phenobarbital at all because they may cause adverse interactions and/or reduce its effectiveness.

These drugs include:

  • Certain anticoagulants
  • Estrogen agents
  • Progestins
  • Beta blockers
  • Corticosteroids
  • Tricyclic antidepressants

On the other hand, some drugs can interact with phenobarbital and increase its effects.

These drugs include:

  • Central Nervous System (CNS) depressants
  • Antihistamines
  • Opiate agonists

Other drugs that may react when combined with phenobarbital include:

  • Beta adrenergic blockers
  • Phenothiazine
  • Aminophylline
  • Chloramphenicol
  • Doxycycline
  • Furosemide
  • Griseofulvin
  • Metronidazole
  • Phenytoin sodium
  • Quinidine
  • Rifampin
  • Theophylline
  • Valproic acid

What Are the Side Effects of Phenobarbital in Dogs?

Phenobarbital is considered an “extra-label” drug, which means it can be used legally by your vet to treat your dog’s seizures, but that it is not FDA approved for that purpose. And unfortunately, as with any drug, phenobarbital does trigger some nasty side effects in our canine friends.

Because phenobarbital works by decreasing neuronal activity, not only does it have effects on the neurons that cause seizures, it also affects other neurons in the body as well. When this happens, your dog can suffer from many side effects related to the function of these neurons.

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Excessive Hunger and Thirst

Dogs on phenobarbital can sometimes experience excessive hunger and thirst. If you notice your dog seems to be eating more than normal and drinking a lot, it’s probably a response to the medications.

Excessive Urination

Sometimes a dog may seem to need to urinate more frequently than normal when he is on phenobarbital. This may be due to his excessive drinking habits. More water in equals more water out.

Weight Gain

Weight gain in dogs could also be attributed to a voracious appetite. If they are eating more than normal, then they could also end up putting on a few pounds, especially if they are also feeling lethargic and not getting much exercise.

Hyperexcitability and Restlessness

In contrast to the lethargic effects phenobarbital can sometimes have on a dog, they can also experience the other end of the spectrum, presenting as restless and hyperexcitable. Dogs that appear to be high strung, pace a lot, pant for no discernible reason, whine or otherwise vocalize, or exhibit other signs of restless or excited behavior may be related to phenobarbital.

Ataxia

Ataxia is when a dog loses coordination in his hind end or experiences weakness there. Phenobarbital can trigger ataxia, and if you notice this, you should let your vet know.

Depression

One of the side effects of phenoarbitral is depression in dogs. Dogs that are depressed may become listless or disinterested in normal activities, and may sleep a lot.

Lethargy

Sometimes a dog can feel lethargic on phenobarbital. If you notice your dog seems to be lying around a lot, moving slowly like he is “out of it”, or behave as though he has recently been sedated, this is most likely due to his levels of phenobarbital and its effects on his neurological system.