Peppermint oil, listed in some products as menthol, is toxic to cats when ingested or inhaled. Of note, some formulations (like wintergreen oil) may contain aspirin derivatives, making them even more dangerous for cats. Birds are also indicated for this toxicity.
Signs and symptoms of toxicity: Signs of toxicity may include nausea, vomiting, altered mental status, and fatigue. There is a risk of liver failure.
Toxic consumption: There is no established threshold for toxicity. All incidents of exposure should be reported immediately.
Osweiler, G, et al. (2011). Blackwell’s five-minute veterinary consult clinical companion. Small Animal Toxicoloty. [Kindle version]. Retrieved from Amazon.com