Cinnamon consumption can cause allergic reactions in some individuals, and the anticoagulant properties of the cassia variety can be problematic, according to the National Center for Complementary and Integrative Health. However, it is generally safe to consume up to 6 grams of cinnamon a day for up to six weeks.
Excessive consumption of cinnamon can irritate the lips and mouth, resulting in sores, reports WebMD. Large quantities of cassia cinnamon can lower blood sugar by too much or cause damage to the liver, warns MedlinePlus. Individuals who use diabetes medications or medications that can harm the liver should be cautious with their intake of cassia cinnamon.