Phenobarbital Side Effects in Dogs

icon June 7, 2024

Phenobarbital is one of the most commonly prescribed medications for managing epilepsy and other seizure disorders in dogs. While it is effective in controlling seizures, phenobarbital comes with a range of potential side effects that dog owners need to be aware of. This article explores the side effects of phenobarbital in dogs, the duration for which a dog can safely stay on the medication, considerations for water intake, guidelines for discontinuing the drug, and the possibility of dogs developing a tolerance to phenobarbital.

Phenobarbital Side Effects in Dogs

Phenobarbital is a barbiturate that works by depressing the central nervous system, thus reducing the likelihood and severity of seizures. However, like any medication, it can cause various side effects, some of which may be serious. The side effects can be broadly categorized into short-term and long-term effects.

Short-Term Side Effects:

♦  1. Sedation and Lethargy:
One of the most common side effects, especially when starting treatment, is sedation. Dogs may appear excessively sleepy, lethargic, or less active than usual.

♦  2. Ataxia:
This is a lack of coordination that can manifest as stumbling, clumsiness, or an unsteady gait.

♦  3. Increased Appetite:
Dogs may exhibit an increased appetite and a tendency to gain weight.

♦  4. Increased Thirst and Urination:
Known as polydipsia and polyuria, dogs may drink more water and urinate more frequently.

♦  5. Anxiety or Hyperactivity:
Some dogs may experience the opposite of sedation, becoming more anxious or hyperactive.

♦  6. Nausea and Vomiting:
Gastrointestinal disturbances such as nausea and vomiting can occur.

Long-Term Side Effects:

♦  1. Liver Damage: Prolonged use of phenobarbital can lead to liver damage or failure. This is one of the most serious potential side effects, necessitating regular liver function tests.

♦  2. Blood Disorders: Long-term use can cause anemia, thrombocytopenia (low platelet count), and leukopenia (low white blood cell count).

♦  3. Behavioral Changes: Prolonged use can lead to significant changes in behavior, including increased aggression or depression.

♦  4. Physical Dependence: Long-term use can lead to physical dependence, meaning abrupt discontinuation can cause withdrawal symptoms.

♦  5. Pancreatitis: There is a risk of inflammation of the pancreas, which can be life-threatening.

How to Minimize the Side Effects of Phenobarbital?

To minimize the side effects of phenobarbital in dogs:

1. Regular Monitoring:
Schedule frequent veterinary check-ups to monitor liver function and blood counts.

2. Dosage Adjustments:
Ensure the dosage is adjusted based on your dog's response and side effect severity.

3. Balanced Diet:
Provide a healthy diet to support overall health and manage weight gain.

4. Hydration:
Ensure constant access to fresh water to counteract increased thirst and urination.

5. Supplemental Medications:
Use additional medications or supplements as recommended by the vet to support liver health.

6. Gradual Changes:
Implement gradual dosage changes to avoid withdrawal symptoms.

How Long Can a Dog Stay on Phenobarbital?

Dogs can stay on phenobarbital for an extended period, often for their entire life, if it is required to manage chronic seizure conditions. The key to long-term use is careful and consistent monitoring. Regular veterinary check-ups are essential to monitor liver function, blood cell counts, and overall health. The dosage may need adjustment over time based on the dog’s response to the medication and any side effects that develop.

Can Dogs Drink Water While on Phenobarbital?

Yes, dogs can and should drink water while on phenobarbital. In fact, increased thirst is a common side effect of phenobarbital, so it is important to ensure that your dog has constant access to fresh water. Adequate hydration is crucial, especially since phenobarbital can cause increased urination, which could potentially lead to dehydration if the dog does not drink enough water.
Also Read: How to Get A Dog To Drink Water

Can You Discontinue Phenobarbital in Dogs?

Discontinuing phenobarbital should always be done under the guidance of a veterinarian. Abruptly stopping the medication can lead to severe withdrawal symptoms and a rebound in seizure activity. If there is a need to discontinue phenobarbital, a gradual reduction in dosage over a specified period is necessary to safely wean the dog off the medication. This process should be closely supervised by a veterinarian to manage any potential side effects and ensure the dog’s safety.

Do Dogs Build Up a Tolerance to Phenobarbital?

Yes, dogs can build up a tolerance to phenobarbital over time. This means that the initial dose that was effective in controlling seizures may become less effective, requiring a higher dose to achieve the same level of control. Tolerance can develop as the dog's body becomes accustomed to the drug, necessitating careful monitoring and potential adjustments in dosage. Regular blood tests are also important to ensure that the drug levels remain within the therapeutic range and to check for any signs of toxicity.


Phenobarbital is a crucial medication for managing epilepsy and other seizure disorders in dogs. While it is highly effective, it also comes with a range of potential side effects, both short-term and long-term. Understanding these side effects, monitoring your dog’s health, and maintaining regular veterinary check-ups are essential components of managing a dog on phenobarbital.

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