Borax and other boron compounds are toxic to cats, although they are also sometimes used as flea treatments on cats. Borax and related compounds are insecticides. Borax has low toxicity when it is ingested, but it is still considered dangerous; the only cure for poisoning is dialysis.
Borax is considered somewhat less toxic than other insecticides, but it can still cause serious problems if moderate to large amounts of it are consumed by a pet. The main danger is if it is liberally sprinkled about to prevent insect infestations. Symptoms of borax poisoning include vomiting, diarrhea, weakness, convulsions and an inability to stand. Cats that are severely poisoned may go into a coma or die. Unlike other poisons, borax poisoning does not respond to activated charcoal.