Some essential oils that are relatively safe for people are toxic to dogs and/or cats.
Essential oils are not very good at killing fleas. We see many animals with fleas that are on these products.
The main reason we hear from clients who want to use such “natural” products is that the products that veterinarians sell are “toxic”, and that their favorite multi-level-marketing essential oils are much safer. We wish we knew how they know that, when our oral products have been tested by the FDA’s extreme safety and efficacy standards, and the topical products we recommend have still been tested by the EPA for safety, and have tons of both safety and efficacy data. We have not seen any evidence of any “natural” products, whether essential oil or not, being tested for either safety or effectiveness.