Many of you know that Phenobarbital is one of the most effective anticonvulsant drugs (AED) that we use to help control feline seizure disorders. Data we published several years ago suggested that 93% of cats obtained seizure control with serum drug concentrations between 15-40 ug/ml, regardless of the etiology. Remember, control is either a 50% reduction in seizures or 1 or less seizures every 3 months. Despite its high accolades for control, phenobarbital can cause adverse effects in cats. Phenobarbital does undergo hepatic metabolism but, unlike in the dog, it does not result in an increase in ALP. So, what adverse effects can happen?
Clinical Adverse effects
Clinical adverse effects occur in 46.7%of cats in a 2021 study (Marsh et al). These may be described as Type I or Type A adverse effects because they are secondary to the drug properties and often dose dependent. Keep that in mind! Sedation and ataxia were the most common side effects, but not the only ones.
Here are the adverse effects, and percent of cats affected, as reported in Marsh's study:
a. Sedation 89%
b. Ataxia 53%
c. Polyphagia 22%
d. Polydipsia 6%
e. Polyuria 6%
f. Anorexia 6%
Perhaps the last 4 are only notable to the observant owner, or in single cat households. Also of note, side effects in cats are reported less often compared with dogs. Interestingly, 26 of the 36 cats with reported adverse effects had transient signs. The majority of these only occurred in the first 4 weeks of treatment and in cats receiving multiple anticonvulsants. Adverse effects were significantly more likely in this population of cats when more than 1 AED was being used.
Biochemical Adverse Effects
A more severe, and concerning type of adverse effect is Type B. These adverse effects were extremely rare in the recent study, as well as in my experience. Bone marrow suppression did occur in 1 cat (as can be seen with dogs) and it resolved with removal of the phenobarbital. Biochemical adverse effects are much rarer in cats than in dogs. If a cat has an elevated ALP, as a general rule, find a reason other than phenobarbital administration. In both the study we published several years ago, and the study by Marsh et al, no cats showed an elevated ALP on laboratory results.
Take home message:
Talk with owners about the common adverse clinical effects in the first 2-4 weeks of treatment. These should resolve but if not, consider reducing the dose.
Recheck the CBC and biochemistry within 4 weeks of starting phenobarbital to monitor for those rare Type B adverse effects.
Phenobarbital is useful, but imperfect, so always look at the cat in front of you when making decisions about seizure management. When in doubt -reach out!
Thanks for reading! I hope you have a good week and are appreciating the slow shift to cold pre-winter weather that we’ve been having. I look forward to working with you soon.