Coumarin is an aromatic organic compound, C9H6O2. It is a colorless crystalline solid with a sweet odor reminiscent of vanilla and a bitter taste. It occurs naturally in many plants, where it serves as a defense against predators, notably tonka bean, vanilla grass, sweet woodruff, sweet grass, sweet-clover, and cassia cinnamon. Ceylon cinnamon does not contain coumarin.
Coumarin is moderately toxic to the liver and kidneys. The German government recommends a tolerable daily intake of 0.1mg coumarin per kilogram of body weight; one teaspoon of cassia cinnamon contains between 6 and 12 mg of coumarin, so the daily dose may be exceeded with that amount of cassia for smaller people.
Note that coumarin is not the same as the prescription drug coumadin, which is a brand for warfarin, a powerful anticoagulant.