Using cinnamon have side effects for some people

Irritation and allergies : Cinnamon usually causes no side effects. But heavy use could irritate your mouth and lips, causing sores. Some people are allergic to it. It might cause redness and irritation if you put it on your skin. Toxicity : Eating lots of cassia cinnamon could be toxic, especially if you have liver problems. Coumarin, an ingredient in some cinnamon products, can cause liver problems, but the amount you’d get is so small that it probably won’t be a problem. Given the lack of evidence about its safety, children, pregnant women, and women who are breastfeeding should avoid cinnamon as a treatment. Lower blood sugar : Cinnamon may affect your blood sugar, so if you have diabetes and take cinnamon supplements, you might need to adjust your treatment.  
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Interactions:  If you take any medication regularly, talk to your doctor before you start using cinnamon supplements. They could affect the way antibiotics, diabetes drugs, blood thinners, heart medicines, and others work.