When speaking about cinnamon, it is important to remember that Chinese cinnamon (Cassia genus) above a certain dose may be toxic because of high concentrations of coumarin, whereas Ceylon cinnamon (genus cinnamonum) ha usually low concentrations of coumarin.
In a few words, if we don't know the origin, it's better we only consume modest amounts of it. This aspect also may pave the way to frauds, due to the lower price of the CAssia, or Chinese variety.
From Wikipedia
https://en.wikipedia.org/wiki/CinnamonToxicity
Further information: Coumarin
A systematic review of adverse events as a result of cinnamon use reported gastrointestinal disorders and allergic reactions as the most frequently reported side effects.[60]
In 2008, the European Food Safety Authority considered the toxicity of coumarin, a component of cinnamon, and confirmed a maximum recommended tolerable daily intake (TDI) of 0.1 mg of coumarin per kg of body weight. Coumarin is known to cause liver and kidney damage in high concentrations and metabolic effect in humans with CYP2A6 polymorphism.[61][62] Based on this assessment, the European Union set a guideline for maximum coumarin content in foodstuffs of 50 mg per kg of dough in seasonal foods, and 15 mg per kg in everyday baked foods.[63] The maximum recommended TDI of 0.1 mg of coumarin per kg of body weight equates to 5 mg of coumarin (or 5.6 g C. verum with 0.9 mg coumarin per gram) for a body weight of 50 kg. C as shown in the table below:
