Phenobarbital is one of the most commonly prescribed anti-seizure medications, particularly for idiopathic epilepsy. It is generally well tolerated in both dogs and cats, though some mostly temporary side effects may occur.
While your pet is on Phenobarbital, regular monitoring is important. Drug levels are checked about 3–4 weeks after starting treatment or changing the dose, and then typically every 6–12 months. At the same time, routine bloodwork (CBC and chemistry panel), along with a physical and neurologic exam, is performed to ensure your pet is tolerating the medication well.
Phenobarbital is a controlled substance, which means it is tightly regulated and can only be prescribed by a veterinarian with a DEA license.
Dosing
This medication should be given twice daily, approximately every 12 hours. Your neurologist may adjust the dose based on drug levels and how well your pet’s seizures are controlled. While the goal is to maintain a 12-hour schedule, the medication’s half-life allows for a bit of flexibility.
In rare cases where seizure control is difficult, your neurologist may recommend giving it three times a day to help improve seizure management.
Possible Side Effects
These are some the transient side effects that typically resolve within 4–6 weeks:
- Sedation or wobbliness: Most pets may be wobbly when walking at first, but their coordination usually improves as they adjust to the medication.
- Increased thirst and urination: Your pet may drink more water and need to urinate more frequently. Do not restrict water, especially during the day.
- Increased appetite: Some pets may have an increased appetite, which can lead to weight gain. Monitor their food intake—just because they seem hungry doesn’t mean they should get extra food. Healthy treats like vegetables are a good option. Some pets may also develop inappropriate eating behaviors, such as chewing toys or getting into garbage.
- Anxiety or agitation: Uncommonly, some pets may show increased nervousness or restlessness.
Other, much less common but more serious side effects can include:
- Liver problems: Rarely, this medication may damage the liver and causes a rapid increases in liver enzymes (ALT). This usually resolves once phenobarbital is gradually tapered off.
- NOTE: This medication is metabolized by the liver, which may result in an increase in a liver enzyme called alkaline phosphatase (ALP). ALP is an inducible enzyme, meaning that an elevated level does not necessarily indicate liver damage. If there is concern about potential liver issues, additional testing such as performing a bile acids test will be recommended to assess liver function more accurately.
- Immune-mediated blood disorders: Some pets may develop anemia, low white blood cells, or low platelets. These reactions are uncommon and usually occur within the first 6 months of treatment. They are typically reversible if the medication is stopped.
- Skin issues: Phenobarbital may rarely increase the risk of a skin condition called superficial necrolytic dermatitis in dogs.
Additional Information
Phenobarbital is one of the few medications that can cause withdrawal symptoms if discontinued abruptly, so it should always be tapered gradually over several weeks. If you and your neurologist decide that tapering is appropriate, they will create a safe, gradual plan for your pet.