Great article on cinnamon on marksdailyapple.com
Smart Spice: Cinnamon
“Note that Cassia (common cinammon) contains significant amounts of coumarin, which humans metabolize to 7-hydroxycoumarin, a toxin moderately damaging to the liver and kidneys. Rodents metabolize it to 3,4-coumarin epoxide, a highly toxic compound, making coumarin a common ingredient in rodenticides. A teaspoon of Cassia cinnamon powder contains 5.8 to 12.1 mg of coumarin and, according to the European Food Safety Authority, the tolerable daily intake for humans is 0.1mg/kg body weight, meaning a daily teaspoon might exceed the limit for smaller individuals. The German Federal Institute for Risk Assessment has gone on record in cautioning against high daily intakes of coumarin (PDF).“
I eat it on my breakfast every day, for the anti-inflammatory effect. Tumeric is good for this too.