PHENOBARBITAL
Phenobarbital (sometimes abbreviated as Pb or Phb) is the most commonly used medication used to treat seizures in dogs because it is relatively inexpensive, easy to administer and effective in 60% to 80% of dogs with idiopathic canine epilepsy if serum concentrations are maintained within the target range. In addition to being used on a daily basis to prevent seizures, Phenobarbital can be used to stop seizures in progress.
Phenobarbital comes in liquid or tablet form. Some
manufacturers use grams and some use grains as a unit of measure. In
milligrams, Phenobarbital is available in 15 mg, 30 mg, 60 mg or 100 mg
tablets. Manufacturers who use grains offer 1/8 grain (8 mg), 1/4 grain
(16 mg), 1/2 grain (32 mg) or 1 grain
(65 mg). Phenobarbital's peak activity occurs 4-8 hours after the pill is given. When
Phenobarbital is started, it takes 1 to 2 weeks to reach a stable blood level and cannot be fully relied upon to prevent
seizures until this period has elapsed.
Dosage:
Because of the variability in absorption, distribution and speed of metabolism among dogs, published dose recommendations only serve as a general guide. Most new patients are started at the lower end of the dose range; however, patients with frequent or severe seizures are often best managed by starting at the higher end of the dose range or by using a loading dose.
The initial starting dose that is frequently used is 2 mg to 3 mg per kilogram of body weight every 12 hours (to convert your dog's weight to kilograms, divide the weight by 2.2 or see conversion chart). Your veterinarian will adjust this dosage based on blood serum levels, seizure activity and side effects of the medication.
Monitoring Therapy:
Therapeutic monitoring of serum drug concentrations are very helpful in
determining the optimal dose of Phenobarbital. Your veterinarian will want to test
the Phenobarbital levels after approximately two weeks of therapy to be sure that your dog
is in a therapeutic range. In most cases a single trough level collected just before
a dose is administered is sufficient. Published therapeutic or target ranges are
only an approximation; however, according to some experts, blood serum levels below 20
mcg/ml are
rarely therapeutic and levels above 35 mcg/ml increase the likelihood of liver disease.
Once a therapeutic range is achieved, blood serum levels should be retested every 6 months
to be sure that the blood concentrations have not drifted out of the intended range.
Side Effects:
Potential side effects include sedation, polyuria (excessive urination), polydipsia (excessive thirst) and polyphagia (excessive hunger), hyperexcitability, ataxia (loss of coordination) and restlessness. Most of these side effects diminish or disappear after the first few weeks of therapy. Polyuria, polydipsia and polyphagia are the most common long-term side effects.
Phenobarbital inhibits seizures by decreasing activity of neurons. Unfortunately, this effect is not specific to the neurons involved in the seizures but affects other neurons as well. Many of the potential side effects of this drug, such as lethargy, ataxia, and weakness, are due to this inhibitory effect on neurons.
Chronic exposure to Phenobarbital can lead to scarring in the liver and liver failure that can be irreversible. This side effect occurs in a small percentage of patients on long term Phenobarbital therapy. Proper monitoring tests, including blood serum for concentration of Phenobarbital and Bile Acid tests are geared for heading off such an event in plenty of time to change medication.
The most common laboratory changes are mild to moderate elevations of serum alkaline phosphatase and other hepatic (related to the liver) enzymes. These changes do not necessarily indicate clinically significant liver disease. Many of our list members have successfully used milk thistle to bolster the liver when their dogs are taking Phenobarbital for seizure control.
Discontinuing Therapy:
The decision to stop therapy must be made very carefully but is reasonable
to consider in dogs that are seizure-free for one to two years. The dose is
gradually tapered over a period of about 6 months. The major risk of discontinuing
drug therapy is seizure recurrence, which is most likely to occur during withdrawal or
within several months of stopping therapy.
Drug Interactions:
Phenobarbital speeds up liver activity and, when used concurrently with other medications that are normally broken down by the liver, may cause them to be metabolized more rapidly, therefore requiring them to be given at a higher dosage. Some of these drugs include antibiotics like chloramphenicol and doxycycline, heart medications like digitoxin, corticosteroids and antiparasitic drugs such as metronidazole (Flagyl). It is also reported to decrease intestinal absorption of the commonly used anti-fungal product, griseofulvin.
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