We had to have our half-Korat put to sleep recently, over something like what you mention. He'd had a severe reaction to a product (not the food in question) and apparently had long term brain damage...he would go from being his loving self to a berserk ball of claws and rage with no provocation. Any other time he was amazingly gentle and lovey...he'd let us use him as a kitty pillow and just purr and purr.
After 3 of those in 2 weeks and he finally sent my mother to the ER for stitches, it was time for the final vet visit. He was only about 4 years old :-(
In the end a product meant to protect him caused all that.
My sympathies.