Phenobarbitol for Dogs

The use of phenobarbitol for dogs that have epilepsy has been widely used, even though the FDA has not approved of such usage. Phenobarbitol (abbreviated as Pb or Phb) inhibits seizures by decreasing the electrical activity of the neurons, especially in the brain. It is easy to use, inexpensive, has a 60% to 80% success rate with idiopathic (without a known cause) epilepsy and unlike most other medications, can stop the shaking mid-seizure.

Brain Chemistry

Epilepsy is caused by the overexcitement of the neurons in the brain. Phenobarbitol reduces this activity, thus preventing seizures. It also increases the activity of gamma-amino butyric acid in the brain that prevents electrical signals from transmitting from one neuron to the next. Unfortunately, it also decreases the activity of all neurons including those not involved with the epilepsy. For this reason, there are several side effects.

Side Effects

As with every medication, there are side effects regarding the administration of phenobarbitol. Some of the side effects include: 

  • Lethargy
  • Excessive thirst
  • Excessive urination
  • Excessive hunger, resulting in weight gain
  • Sedation
  • Hyperexcitability
  • Restlessness
  • Ataxia (loss or coordination or weakness in the hind end region)

These side effects dissipate within a couple of weeks as the dog’s body adjusts to the medication. In rare instances, phenobarbitol can cause anemia, necessitating discontinuing use.

Peaks and Valleys

Due the variance in speed of metabolism, absorption and distribution among patients, the medication peaks and decreases throughout the day after administration of the drug. It is important that the lowest level is “high” enough to prevent a seizure and the highest (peak) level is “low” enough so that it does not cause irreversible liver damage and eventually death. Sometimes phenobarbitol is combined with potassium bromide (milk thistle) to prevent liver damage. Liver damage occurs in a small percentage of dogs. Therefore, it is extremely important to administer the drug as close to 12 hours apart as possible. It takes approximately 1 to 2 weeks for phenobarbitol to stabilize in the blood when first started. If there is more than one seizure every two weeks or seizures are lasting more than five minutes, a veterinarian should be contacted immediately.

Liver Damage

Long-term use of phenobarbitol can produce liver damage. Detected early enough, the damage can be reversed. It is extremely important to monitor the dog’s behavior after every dosage.

Early signs of liver damage include:

  • Weight loss
  • Loss of appetite
  • Chronic intermittent vomiting (common)
  • Chronic intermittent diarrhea
  • Increase thirst (caution: also common with the use of phenobarbitol)
  • Increase urination (caution: also common with the use of phenobarbitol)

Increase thirst and urination are also side effects of phenobarbitol, so care must be taken in distinguishing the difference.

Later signs of liver failure include:

  • Swollen belly
  • Dark brown urine
  • Pale gray or mustard colored stools
  • Lethargy
  • Jaundice (whites of the eyes and mucous membranes of the gums and tongue turn yellow)


There are alternative drugs for treating epilepsy; however, they are more expensive, but do not pose the risk of liver damage.