Drug of first choice in management of canine epilepsy.
In dogs, phenobarbital can be started at 3 mg/kg twice daily orally. Oral doses are adjusted based on serum levels and seizure frequency. Recommended therapeutic range in dogs is 20 to 35 ug/ml.
Common side effects include polyuria, polydipsia, polyphagia, transient sedation or hyperexcitability and ataxia. These are common one to two weeks after the onset of the treatment or after increasing the oral dosage but usually resolve as tolerance develops. Polyuria, polyphagia and polydipsia are the most common long-term side effects. Other side effects include haematological abnormalities such as neutropenia, anaemia and thrombocytopenia.
Hepatotoxicity is rare (mostly seen in dogs on chronic treatment with phenobarbital serum level above 35 mg/L) and is characterised by sedation, ataxia, anorexia, icterus, ascites or coagulopathy with increased pre- and post-prandial serum bile acids, low albumin, increased ALT, ALP, bilirubin. An increase in liver enzyme concentration is to be expected in all animals receiving phenobarbital and so liver function tests are required to monitor the effects on the liver. Hepatotoxicity may be reversible if detected early and phenobarbital withdrawn. There is an increased risk of pancreatitis in dogs receiving both bromide and phenobarbital.
• Phenobarbital treatment does not affect adrenal function tests (ACTH stimulation test and low dose dexamethasone test) despite acceleration in dexamethasone metabolism.
• Phenobarbital treatment significantly decreases total T4 and free T4 concentration. TSH only shows a compensatory increase while total T3 has minimal fluctuation and cholesterol increases toward the upper limits of the normal range.
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Gieger TL, Hosgood G, Taboada J et al (2000) Thyroid function and serum hepatic enzyme activity in dogs after phenobarbital administration. JVIM 14, 277-281. - PubMed -
Monteiro R, Anderson TJ, Innocent G, Evans NP, Penderis J (2009) Variations in serum concentration of phenobarbitone in dogs receiving regular twice daily doses in relation to the times of administration. Vet Rec. 165(19):556-8. - PubMed -
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