Mothballs deter cats, but using them for this purpose is not recommended as they are highly toxic, according to the Pet Poison Helpline. When ingested, mothballs can kill cats, dogs and other pets. The vapor they give off is also quite harmful to most living creatures.
Mothballs release a toxic pesticide vapor that is intended to kill moths and their larvae. They are often used to repel snakes, rodents and small animals, but they are not recommended for this purpose. Cats are very sensitive to the toxic effects of mothballs, and exposure can be life-threatening. Mothballs can cause lethargy, seizures, vomiting, tremors, coma and death in cats.