Welcome to JustAnswer! I am a licensed veterinarian and would be glad to help!
No - this is not commonly used in small animals (cats and dogs)
It is not well tolerated - and at this time the consensus is that carbamazepine should be considered as a 3rd-line agent for use in small animals after the more common options based on response to therapy and (tolerated) side effects.
what is a 3rd line agent please?
meaning not the first or second choice, but the 3rd choice if nothing else is working.
as the epilepsy is idiopathic what dosage of carbamezepine would be needed for a 4.5kg cat?
I would not recommend this drug as a primary epilepsy drug for a cat.