Phenobarbital is the most widely prescribed medication for treating seizures in dogs and is usually the first medication that is given to a dog with epilepsy. Phenobarbital is effective in 60 to 80 percent of dogs with idiopathic epilepsy if serum concentrations are kept within the target range.
Phenobarbital is a barbiturate and a nonselective central nervous system depressant. As we discussed earlier, seizures are caused when the balance of excitation and inhibition in the neurons of the brain is disturbed. Phenobarbital works by increasing the action of GABA (the inhibitory neurotransmitter) and it also appears to inhibit the release of glutamate (an excitatory neurotransmitter) from nerve endings.
Mean Elimination Half-Life: 37 to 73 hours
Time to Reach Steady State Concentrations: 10 to 15 days
Target Serum Concentration: 15 to 40 ug/ml
Adverse Effects: The common side effects of Phenobarbital are excessive hunger, excessive thirst, excessive urination, lethargy and ataxia (hind end weakness). It is not uncommon for some patients to demonstrate depression or sedation when Phenobarbital therapy is initiated. These effects are usually transient and resolve as the patient acclimates to the medication. If these side effects do not resolve, or if they are extremely pronounced, talk to your veterinarian.
Less common, but more serious side effects of Phenobarbital are scarring of the liver and liver failure that can be irreversible. Monitoring liver function while using Phenobarbital is vitally important.
Rarely, anemia (lack of red blood cells) can occur with Phenobarbital exposure.
Monitoring: There are many monitoring protocols but in general, Phenobarbital levels should be checked approximately two weeks after initiating therapy to determine the blood serum levels. Generally a blood sample can be taken at any time, however if your dog continues to have seizures, your vet may want to do a peak (2 to 4 hours after giving the medication) and a trough (right before the next dose is due) serum level. This will help determine what the half life of the medication is for your dog.
Once you have obtained seizure control, your vet will probably want to monitor blood serum concentrations every 6 months to be sure that the serum level has not drifted out of range. The blood level of Phenobarbital attained in an individual is not predictable by knowing the dose given. With time, the patient's liver becomes extremely efficient at removing Phenobarbital from the system and the level may go down. The opposite may also be true and the liver can become less efficient so that blood levels go up. In order to maintain seizure control, it is important to check blood levels periodically.
Very important information follows:
Although the incidence of serious liver damage caused by phenobarbital is small, it can and does happen. In addition to monitoring blood serum levels it is extremely important to check liver function so that any damage is caught while it is still reversible. Phenobarbital is known to increase liver enzymes so it is not uncommon to see elevations in AST, ALP and ALT on a blood chemistry test. This makes interpretation of a chemistry panel difficult. Bile acid tests are not falsely affected by Phenobarbital and are a much more sensitive and accurate way of testing liver function. Most veterinarians recommend bile acid tests be run at least every 6 months. Personally, I run a bile acid test on Radar every 3 to 4 months because I have heard of cases where liver disease struck very hard and fast. Early detection of liver failure is critical because in the early stages it is almost always reversible.
The last test that should be performed on a regular basis (every 6 months) is a CBC or complete blood count to be sure that there is no anemia present.
Please see the section on laboratory tests for information about chemistry panels, bile acid tests and complete blood counts.
Cautions and Warnings: Phenobarbital should be used cautiously in dogs who are anemic, have borderline hypoadrenal function, liver disease or cardiac or respiratory disease.
It is very important to comply fully with the treatment plan that your vet has recommended as missing even one dose of Phenobarbital can be enough to trigger a seizure.
Discontinuing Therapy: Phenobarbital should never be suddenly withdrawn except under extreme circumstances. In those instances where Phenobarbital must be withdrawn quickly, it is best to hospitalize the patient so that any adverse effects can be treated promptly.
Reducing Phenobarbital Doses: In most cases, it's important to reduce Phenobarbital doses slowly. Reduce the current dose by 20% wait about two weeks and repeat. Sudden reductions in Phenobarbital doses may cause severe seizures.
Berendt, M, Clinical
Neurology in Small Animals-Localization, Diagnosis and Treatment
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Last Updated August 2009