Irritation of the mouth, lips and skin are possible side effects of consuming high quantities of cinnamon, according to WebMD. Cinnamon may also lower blood sugar and cause liver problems or allergic reactions. Usually cinnamon does not cause side effects.
A person who ingests large quantities of cinnamon may develop sores on the mouth and lips due to irritation, explains WebMD. If applied to the skin, cinnamon may cause skin reddening or allergic reactions. Cinnamon's ability to lower blood sugar may be significant for diabetics who strive to keep their blood sugar levels within certain ranges. Diabetics need to keep close tabs on their blood sugar levels when they consume high levels of cinnamon. They may need to adjust the amount of medications they take or reduce their cinnamon intake. Low blood sugar is also a concern during surgical procedures, so individuals expecting to undergo surgery must avoid cinnamon for two weeks prior to the procedure.
Some types of cinnamon contain coumarin, a chemical that can cause or exacerbate liver disease when a person ingests large quantities, notes Sandi Busch for SFGate. It is especially important to limit use of certain types of cinnamon when taking medications that have the potential to harm the liver, states WebMD. These medications include acetaminophen and erythromycin, among many others.