While flea and tick products containing pesticides repel and kill parasites that can potentially transmit disease-causing organisms, they can be harmful to your pet, your child and even adults in your household. It is not without reason that they shouldn’t be used on debilitated, aged, pregnant or nursing animals. Their potential toxicity and a range of harmful effects is obvious, as these products must be applied in the only place the dog is unable to lick or bite – it’s back.