Although cinnamon supplements generally cause no side effects, excessive use of cinnamon may cause irritation of the lips and mouths, resulting in sores, according to WebMD. Some users may experience allergic reactions, and cinnamon often causes toxicity in people with liver problems.
One of the side effects of cinnamon is that it lowers blood sugar, so diabetics taking cinnamon supplements often need to adjust their treatment plans, notes WebMD. In addition, coumarin, an ingredient in cinnamon products, may cause liver problems. Cinnamon may also interact with diabetic drugs, antibiotics, heart medications and blood thinners, contraindicating its use in people using those medications. Because a lot remains unknown about cinnamon supplements, experts do not recommend it for children or pregnant or lactating women.