Phenobarbital


Background

When animals have frequent seizures, medication becomes necessary. In dogs and cats, the barbiturate Phenobarbital is the first choice for seizure suppression. It is effective, safe if used responsibly and is one of the least expensive medications in all of veterinary practice. Since treatment with Phenobarbital is generally life long, monitoring blood tests are periodically recommended.  Most often Phenobarbital is used to suppress epileptic seizures, but Phenobarbital can also be used against seizures due to brain tumours, poisonings, or infections as well.

When Phenobarbital is started, it takes 2-4 weeks to reach a stable blood level and can not be fully relied upon to prevent seizures until this period has elapsed. Usually a blood Phenobarbital level is run at the end of this period to determine how the pet is absorbing the medication.

Side Effects

  • Excessive thirst and urination and excessive appetite have been reported.
  • Some patients demonstrate depression or sedation when Phenobarbital therapy is initiated.
  • Chronic exposure to Phenobarbital can lead to scarring in the liver and liver failure that can be irreversible.

Interactions with other Drugs

The following drug interactions have either been reported or theoretical in animals receiving Phenobarbital:

  • Acetaminophen
  • Monamine Oxidase Inhibitors
  • Phenytoin
  • Rifampin

Some medications may increase the effects of Phenobarbital:

  • Antihistamines
  • Chloramphenicol
  • Opiates
  • Phenothiazines
  • Valproic Acid

Concerns and Cautions

Phenobarbital is able to induce the metabolic enzymes, thus making them more efficient at removing toxins. Part of this phenomenon involves elevation of liver enzyme tests on a panel. As previously mentioned, monitoring by periodic blood testing is important in catching any impending liver problems while they are still insignificant.

There are many monitoring protocols. Hospitals will recommend a Phenobarbital blood level drawn at the time of day when the level is the lowest (a trough level) 2 to 4 weeks after starting Phenobarbital followed by a similar Phenobarbital level test every 6 months. We also run a liver function test called bile acids test every 6 months in addition to the Phenobarbital level test.

Veterinarians Comment

MISSING EVEN ONE DOSE OF PHENOBARBITAL CAN BE ENOUGH TO TRIGGER A SEIZURE. IT IS IMPORTANT TO COMPLY FULLY WITH MEDICATION DIRECTIONS.