Verdict: In moderation
Cinnamon is generally not toxic to cats in very small amounts, but it can cause oral irritation, digestive upset, and respiratory issues if inhaled. Large quantities, especially concentrated forms like essential oils, can lead to more severe symptoms including liver damage.
Irritation of mucous membranes; potential liver enzyme inhibition with high coumarin intake; essential oils directly toxic to liver.
Coumarin, Cinnamaldehyde, Eugenol (in some varieties)