Side effects of heavy cinnamon consumption include potential sores due to irritation of the mouth and lips and an allergic reaction in some cases, according to WebMD. Very high doses of cassia cinnamon may be toxic, primarily to those with liver problems.
Though cinnamon generally does not cause side effects, there are some risks associated with cinnamon usage in certain situations, according to WebMD. Those with diabetes may need to alter their treatment if they use cinnamon supplements, because cinnamon may lower fasting blood sugar. Some cinnamon products contain coumarin, which may cause liver problems. WebMD does not recommend that children and breast-feeding or pregnant women use cinnamon as a treatment due to the lack of evidence about its safety.