What is a side effect of cinnamon?

As reported by WebMD, one side effect of cinnamon is that it may cause or worsen liver disease. Because cinnamon contains the chemical coumarin, liver failure may occur if taken daily or in high doses.

Cinnamon can also lower blood sugar, but it might have an adverse effect and interfere with blood sugar control during and after surgical procedures, according to WebMD. It is recommended to stop taking cinnamon at least two weeks before a scheduled surgery. Pregnant women should not take cinnamon because it may induce premature labor or uterine contractions. Women who are breast feeding should also err on the side of caution and avoid cinnamon.