(for veterinary information only)
When an animal has a seizure disorder, the frequency and severity of the seizures determines the need for treatment. In the dog and cat, phenobarbital, a barbiturate, is probably the first choice for seizure suppression. It is effective, safe if used responsibly, reasonably priced, and conveniently dosed all of which makes it a popular choice. Since treatment with phenobarbital is generally life-long, monitoring blood tests are periodically recommended.
Phenobarbital is generally effective regardless of the underlying cause of the seizure disorder which means that is can be used for epilepsy, brain tumors, infectious diseases, or poisonings.
HOW THIS MEDICATION IS USED
Monitoring blood tests are important for long term use of phenobarbital (see below).
COMMON POSSIBLE SIDE EFFECTS OF PHENOBARBITAL
Sedation: The goal of seizure therapy is to end the seizures, not sedate the patient. Most patients will not show noticeable sedation on phenobarbital and if they do, there is a good chance the dose is too high. Some patients cannot find an effective dose of phenobarbital that is not sedating and these patients may need a different medication (or simply a reduction of the phenobarbital dose and addition of a second medication).
Sedation During Initial Therapy: It is not unusual for some patients to demonstrate depression or sedation when phenobarbital therapy is initiated. This effect is generally transient and resolves as the patient acclimates to the medication. If this problem has not resolved after two weeks, a blood level of phenobarbital will determine if the dose is simply too high and requires adjustment.
Excessive Appetite: Phenobarbital makes patients very hungry. This can be problematic if the patient becomes obese or becomes overly focussed on food.
Excessive Thirst: Phenobarbital makes patients thirsty. This could become a problem if the patient drinks so much that excessive urine is produced leading to house-soiling.
UNCOMMON POSSIBLE SIDE EFFECTS OF PHENOBARBITAL
Chronic exposure to phenobarbital can lead to scarring in the liver and liver failure that can be irreversible. Proper monitoring tests are geared for heading off such an event in plenty of time to change medication. Liver toxicity is associated with prolonged phenobarbital blood levels of 35 ug/ml or higher. Liver toxicity appears to be a problem for dogs but not for cats.
Cats can experience facial itching on phenobarbital. This can be problematic enough to require a medication change.
Rarely, anemia (lack of red blood cells) can occur with phenobarbital exposure. Should this occur, a different seizure medication should be selected.
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 blood panel. As previously mentioned, monitoring by periodic blood testing is important in catching any impending liver problems while they are still insignificant but this is complicated because elevations in liver enzymes occur with normal phenobarbital usage.
There are many monitoring protocols. Here are the recommendations from the aforementioned ACVIM Consensus Statement:
The first sample should be run two weeks after beginning medication followed by another blood level six weeks after starting medication. After that blood levels should be checked every 6 months or if the pet has >2 seizures between these times or two weeks following any change in dose. Levels are best drawn just before medication is due to be given so as to get the lowest blood level of the day (a "trough" level).
INTERACTIONS WITH OTHER DRUGS
Phenobarbital has a very long list of potential drug interactions so it is very important to inform your veterinarian about all the drugs and supplements your pet is taking.
As mentioned, in the liver, phenobarbital has a unique ability to “induce” the microsomal enzymes which means in more simple terms that chronic exposure to phenobarbital makes the liver more efficient at removing other toxins.
Medications which will not work as well in the presence of phenobarbital include:
Lysodren (treatment for Cushing’s disease), chloramphenicol (an antibiotic), corticosteroids (such as prednisone, dexamethasone), doxycycline (an antibiotic), estrogens, cardiac beta-blockers, quinidine (a heart rhythm medicine), theophylline (an airway dilator), cyclosporine (for immune mediated diseases), levetiracetam (another anti-seizure drug), praziquantel (dewormer), itraconazole (anti-fungal), doxorubicin (agent of chemotherapy), and metronidazole (a multi-purpose antibiotic/GI medicine).
Phenobarbital’s activity can be enhanced by concurrent administration of the following medications: chloramphenicol (an antibiotic), any antihistamine associated with drowsiness, or any other sedative or tranquilizer.
Rifampin, a special antibiotic, may reduce the effectiveness of phenobarbital when the two are used concurrently.
When unacceptable side effects develop with phenobarbital use, phenobarbital dose may be substantially cut back or even discontinued. Potassium bromide can be used as an alternative or supplemental seizure control medication in such cases.
As mentioned, long term exposure, particularly at higher doses, can induce scarring in the liver and proper monitoring is needed to avoid this side effect. Liver toxicosis is enhanced by the use of the following drugs: rifampin (a special antibiotic), acetaminophen (pain reliever), carprofen (anti-inflammatory pain reliever), selegiline (for cognitive dysfunction), and amitraz (common ingredient in tick protection productions for dogs).
CONCERNS AND CAUTIONS
MISSING EVEN ONE PHENOBARBITAL DOSE
If you accidentally skip a dose, do not double up on the next dose.
If your pet has a break-through seizure, however, you may give an extra dose
In patients with poor liver function or liver failure,
Page last updated: 5/21/2021