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.

side effects of phenobarbital for dogs_canna-pet

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 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.


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.


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.

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Most of the time, these side effects will resolve on their own or diminish the longer your dog remains on the treatment. If any of these side effects do not resolve on their own, or have appeared to become magnified, then you may need to discuss it with your vet and possibly seek alternative medications and therapies to treat your dog’s seizures.

There are also more serious side effects to phenobarbital usage when it’s taken long-term. These side effects can harm a dog’s liver, although thankfully it appears to only affect a small percentage of canines and not the majority.

In those cases, phenobarbital can cause scarring of the liver and liver failure. Liver failure is irreversible. This is the reason it is very important to monitor your dog’s liver function while using phenobarbital, and adjust the levels as needed.

Using this drug frequently over a long period of time can ultimately lead to death because it is so hard on the liver. Also, when phenobarbital is combined with other medications or alternative remedies like milk thistle, it only increases a dog’s chances of developing liver disease. Your dog could develop serious warning signs of liver damage that you need to be aware of and watch for if your dog is taking phenobarbital.

Rapid Weight Loss

Kind of the opposite of gaining weight, but weight loss occurs in dogs that are experiencing damage to their liver. If you notice your dog seems to be dropping a lot of weight while they are taking phenobarbital, see your vet as soon as possible to discuss your options.

Excessive Vomiting

Sometimes when experiencing liver problems, a dog may vomit excessively. You may also notice your dog’s abdomen seems swollen or distended, which can suggest your dog may be in pain. Frequent and excessive vomiting in dogs can lead to other health concerns, like dehydration, so be vigilant and see your vet if you think this may be a problem for your dog.


Jaundice in dogs is a yellowing of the whites of the eyes and skin, and is a clear indicator that your dog’s liver is not functioning optimally. Don’t delay in seeing your vet and seeking treatment.

Dark Urine with a Foul Odor

Impaired liver function can result in your dog peeing very dark urine with a strong, foul odor. If you notice this, see your vet and do not delay.

Pale Gray or Mustard Colored Stools

Sometimes a dog experiencing liver failure may present with funky colored stools such as pale gray or mustard colored. This is a clear warning sign that should not be ignored, because it means that your dog’s liver damage is creeping into irreversible damage territory.


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In even more rare cases, phenobarbital can trigger anemia in dogs. Thankfully it’s not common, but if you suspect your dog may be anemic, tell your vet because anemia can lead to a whole host of other health problems that you do not want your dog to suffer through.

Any of these may be signs that your dog is experiencing liver damage due to the use of phenobarbital, and your dog should be seen as soon as possible to test his blood levels and possibly adjust his dosage.

While some of these conditions may resolve on their own once your dog has adjusted to the phenobarbital in his system, some of these conditions will not, and some of these conditions can indicate a more serious underlying health concern that needs to be addressed.

How Do I Minimize Phenobarbital Side Effects in Dogs?

Some ways you can help limit phenobarbital side effects over the long-term, is to use the minimal viable dose that seems to work for your dog. This is where regular monitoring of your dog and his blood levels and behavior changes is helpful.  

The earlier you can catch any of these signs or symptoms, the better your chances of adjusting the medication levels and treating your dog successfully while avoiding the brunt of nasty side effects and related health concerns.

When Should I Stop Using Phenobarbital in Dogs?

Only you can decide when to take your dog off phenobarbital. If your dog has been free of seizures for at least one or two years, he might be an ideal candidate to stop phenobarbital treatment.

This is especially viable if your dog is suffering from a lot of the side effects and it is hindering his quality of life, or if you suspect your dog may be experiencing liver damage. Better to stop the drug early before any liver damage becomes irreversible.

However, keep in mind that phenobarbital should never just be stopped cold turkey, as stopping cold turkey could trigger more seizures. Even when you take your dog slowly off phenobarbital under the supervision of your vet, your dog could still experience seizures during the withdrawal phase.  

Alternatives to Phenobarbital in Dogs

In some cases, you might choose to forgo using phenobarbital at all with your dog. There’s nothing wrong with making that choice, although it may not be recommended by your vet.

There are dietary changes that may be helpful in treating a dog with seizures, but there is no definitive science behind it, at least with dogs. Some pet owners choose to try herbal remedies like milk thistle and potassium bromide.

However, alternative remedies should be used with just as much caution as medications like phenobarbital, because they are not always effective at treating the problem, and can sometimes come with their own set of side effects. You know your pet best and what seems to work for him as opposed to what doesn’t. The side effects of phenobarbital in dogs can be grim, but for some dogs, it may be their only choice. Only you can decide.


  1. “Phenobarbital.” PetMD, Accessed 13 Oct. 2017.
  2. Clark, Mike. “Phenobarbital For Dogs: Uses, Dosage, & Side Effects.” Dogtime, 23 Oct. 2017, Accessed 13 Oct. 2017.
  3. “Phenobarbital Side Effects in Dogs.” VetInfo, Accessed 13 Oct. 2017.
  4. Schamble, Melody. “Phenobarbital for Dogs with Seizures.” American Kennel Club, 22 Aug. 2016, Accessed 13 Oct. 2017.
  5. Huston, Lorie. “What to Know About Phenobarbital Seizure Treatment for Dogs and Cats.” The Spruce Pets, Accessed 13 Oct. 2017.

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