We see a lot of reactions to otc flea products in dogs and cats. Most people think they are the same as frontline and advantage-NOT. I even use frontline on my chickens for mites and lice. Most of the otcs are pyrethrins or permethrins which actually are natural products but can be dangerous in animals sensitive to them. They are derivatives from the mum plant. If your vet has contacted them about the reaction they should pay the bill, the times we have had reactions to products the companies have footed most if not all the bill.