Cats are notoriously sensitive to toxins; their livers are not able to metabolize many substances which may safely be used on dogs (cats have been poisoned through use of dog flea preparations). For this reason, a substance shown to be beneficial and safe for humans may be unsuitable for use on cats. Tea Tree oil (melaleuca, Melaleuca alternifolia) is a phenol-containing essential oil. Its active ingredients are cyclic terpenes which have a similar structure and action to turpentine (a known toxin) - Cats are uniquely sensitive to phenolics and other benzene-based compounds. Benzyl alcohol (a preservative) is also toxic to cats.