Like many other flea and tick medications, PetArmor too has certain side effects. These effects are usually temporary, but you should monitor your cat for prolonged side effects. Get in touch with your vet if these side effects continue for a long period, say a week or two. PetArmor side effects include skin irritations like discomfort, redness, scratching, and so on. Vomiting and diarrhea are also common gastrointestinal problems that are seen as side effects of this drug. Stop applying the medication if you notice any of these side effects, and contact your vet to know about the next course of action.