Are Essential Oils bad for dogs and cats? I used to use Essential oils in a diffuser for my own health all the time. But since hearing from some sources that these are bad for animals I have stopped. I am getting contradicting information from animal professionals as to if using essentials oils in a diffuser is bad for animals. I have 5 cats and two dogs and do not want to do anything to upset them. One of my cats has IBS but all the other animals are healthy if that is relevant. If essential oils are not bad for animals I would love to start using them again. Anyone know for certain?