Poisoning associated with topical medications is uncommon in dogs and cats. The topical product usually associated with toxicity is an inappropriately applied topical flea product. The products specifically labeled for use in dogs can result in serious toxicity if administered to cats. The toxic substance in these products is permethrin, which can have devastating effects if given to cats.
For more information, read the article Permethrin/Pyrethrin Toxicity in Cats or Permethrin/Pyrethrin Toxicity in Dogs.