Borax, a boron mineral common in cleaning products, is sometimes recommended for helping rid a house of fleas, because the natural insecticide kills fleas by perforating their exoskeletons. However, borax can be toxic to animals. Don't use it without consulting your vet.
Using borax for household flea removal requires dusting your floors and carpets, which puts your cat at risk of inhaling the fine powder, or walking through it and later ingesting it while grooming. Your cat's reaction will depend on how much borax your cat gets in his system as well as on how it got there and where it entered, and on your cat's sensitivity level.
Toxicity Side Effects
A cat experiencing borax poisoning may have skin rashes and swelling, eye and mouth irritation, vomiting, nausea and breathing problems. Call your veterinarian right away. Additional signs of poisoning in general are increased thirst, increased urination, seizures, fast heartbeat, depression and lethargy.
What to Do After the Fact
If you see something unusual on your cat's fur and suspect that he got into borax or any other type of poisoning, try to keep him away from any other household pets. Try not to let him groom himself, and wash his fur gently with mild shampoo. Call your veterinarian. She may suggest that you induce vomiting or that you bring your cat in for a visit.
Borax vs. Boric Acid
Don't confuse borax with boric acid. While they are closely related, they are not the same. Boric acid is a type of refined borax with additional ingredients; it is labeled as a pesticide. Borax is not labeled as a pesticide. The toxicity levels of borax and boric acid are considered very low for people but not necessarily for animals.