While pure essential oils are generally safe when used appropriately, it's wise to exercise particular caution with specific "hot" oils or those high in phenols/eugenols for direct topical or internal use, especially with cats and very small dogs. These include oils like Birch, Melaleuca (Tea Tree), Wintergreen, Oregano, Cassia, Cinnamon, Clove, Peppermint, Rosemary, and Thyme. Diffusion of these oils remains a gentler alternative.