Phenobarbital is the most commonly recommended anticonvulsant drug to control epilepsy in cats. It is inexpensive, has an excellent pharmacokinetic profile, and does not appear to cause hepatic enzyme induction or have the same hepatotoxic potential in cats as it does in dogs.3,5,6 Anticipated adverse effects are usually mild and transient and consist of increased appetite, thirst, sedation, and ataxia. Generalized lymphadenopathy that resolves on withdrawal of the drug has also been reported in a cat receiving phenobarbital.7 Although q24h dosing may be adequate in some cats, q12h dosing is often recommended to ensure a steady serum level.6,8
The established therapeutic range of serum phenobarbital levels for dogs (15-45 μg/mL) appears to apply to cats as well.2 In general, about 40% to 50% of cats become seizure free on phenobarbital, and an additional 30% to 60% are considered well controlled.3,8
Because it is difficult for many owners to administer oral medications, an alternative route of administration may be desirable. A recent study provided evidence that therapeutic serum levels of phenobarbital can be achieved via transdermal administration.9 Two different bases (pluronic lecithin organogel and Lipoderm ActiveMax; pccarx.com) were used, and serum levels between 15 and 26 μg/mL were achieved by administering each at a dose of 9 mg/kg q12h. However, serum levels varied significantly between the different vehicle formulations at the same dosage.9
As a general note, transdermal drug absorption depends on the molecule size, chemical nature, and dosage; therefore, not all medications can be absorbed through the skin nor are all medications safe to administer transdermally.10 Antiseizure medications that have not been shown to be absorbed transdermally should not be prescribed for this route. Serum levels should be monitored at least every 6 months in cats treated with transdermal phenobarbital to ensure safe and effective absorption, and cats should be monitored for dermatologic reactions on the ear pinnae.9