Yes, it can happen on occasion, we have had one rather violent reaction to frontline in the 8 years I have been there. One of my dobermans is allergic to Frontline but only breaks out in hives for a day or so but I still just use advantix on her even tho the hives don't seem to bother her, she just looks lumpy for a day or two. We just have a LOT more reactions to the over the counter medicines mostly because they are products based on older methods, which was usually a pesticide you put on the animal that could be harmful to people and animals, where as Advantage and Frontline are designed to work on the fleas and ticks and not the dogs yet reactions do happen on occasion. I will say tho that Fort Dodge's new product Promeris (flea and tick) we are not carrying at the vet I work at. We have had LOTS of bad reports on it making animals ill, one of my coworkers used it on her dog as a trial when it first came out for the vet she worked at before coming here. It made her dog's hair fall out where she applied it, her dog was lethargic for a week even after washing the product off when she saw that Molly was getting VERY sick within an hour of application, and on top of this and other issues it caused temporarily they had reports of it not working at all, Megan didn't see any signs of fleas dying in the hour before she washed the Promeris off, in fact she watched them walk right thru the product with no adverse effects. It is my understanding that the main active ingredient in Promeris is one of the older flea and tick drugs that had faded out of use because it caused so many adverse side effects. Anyways, between this and other reports we have gotten we do not carry it, we figure we have enough products at the moment that seem to be working for people, people seem to be happy with them so we dont want to take chances with this new product until Fort Dodge has more time to work out the kinks, if possible.
Sorry if the previous post sounded like you wont ever have any adverse effects with Frontline or Advantage, I just meant they are much less likely. On another note if you ever put anything like this on your dog/cat and they start acting sick one of the best things you can do is wash them right away 2 or 3 times to get the product off, and something like dishwasher detergent, Dawn etc, which is a soap based product is the best to use because it will strip the product. Most pet shampoos are not soap based.