Phenobarbital is an anti-convulsant prescribed for "recurrent and uncontrolled feline seizures," explains VetInfo.com. Phenobarbital is classified as a barbiturate and suppresses seizure activity by relaxing the feline's muscles. Both VetInfo.com and Mar Vista Animal Medical Center agree that phenobarbital is the first choice of vets when prescribing anti-seizure medication (as of this writing in 2010). Mar Vista adds that phenobarbital is also extremely inexpensive--good news to cat owners, because the feline will likely be on it for life. Phenobarbital is not without side effects, though.
Cat owners may notice fatigue or the exact opposite, agitated anxiety, when the feline begins phenobarbital therapy. Fortunately, Doctors Foster & Smith report that this side effect is usually temporary and will dissipate quickly as the cat adjusts to the new medication. If it doesn't go away, they recommend a return trip to the vet.
There is a possibility the cat will be allergic to one or more of the ingredients in phenobarbital. Symptoms of an allergic reaction include "facial swelling, hives, scratching, sudden onset of diarrhoea, vomiting, shock, seizures, pale gums, cold limbs, or coma," per Doctors Foster & Smith. The feline should be taken to the vet immediately if they experience an allergic reaction to phenobarbital.
Phenobarbital is a muscle relaxant; consequently, owners may observe their cat staggering when trying to walk. The cat may also seem depressed or sedated. Mar Vista assures these side effects minize in time as the cat acclimates to the medication. If necessary, dosage may be reduced to ease these side effects.
Phenobarbital is also known to increase the feline's propensity to eat, drink and urinate. Phenobarbital can dehydrate the feline, according to VetInfo.com, which might explain the increased need to drink water and, in turn, urinate.
A rare side effect of phenobarbital is liver damage; as such, animals with liver disease should not take phenobarbital. The liver damage may not reveal itself until the cat becomes jaundiced. When this happens, the feline's skin, gums and whites of its eyes become yellowish in colour.