Coumarin is a flavouring substance which is contained in relatively high concentrations in cinnamon varieties collectively known as “Cassia cinnamon”. In especially sensitive persons, even comparatively small quantities of coumarin can cause liver damage, although the effect is usually reversible. Isolated coumarin must not be added to foods. However, to flavour foods, coumarin-containing plant parts may be used. For cinnamon-containing foods, new maximum permissible coumarin levels have been in place in the European Union since 2011. In contrast, there are no limit values for cinnamon as a spice.
Consumers who take cinnamon-based food supplements should be aware that such products may contain high quantities of Cassia cinnamon. The BfR recommends moderate consumption of Cassia cinnamon. Consumers who frequently use large amounts of cinnamon as spice in their home cooking, for example for rice pudding with sugar and cinnamon, should use Ceylon cinnamon which contains low levels of coumarin.
For more info: faq-on-coumarin-in-cinnamon-and-other-foods
Source: www.bfr.bund.de / Federal Institute for Risk Assessment ( BfR ) – Germany