Cinnamon, from the genus Cinnamomum and Lauraceae family, has been used as a popular spice for thousands of years around the world. Many studies have shown therapeutic effects of cinnamon including its antimicrobial, antiviral, antifungal, antioxidant, antitumor, antihypertensive, antilipemic, antidiabetic, gastroprotective, and immunomodulatory effects. Due to popular use of cinnamon and several human reports on adverse events associated with short or long term use of cinnamon, we aimed to systematically review its human reports of adverse event. Databases including Medline, Scopus, Science Direct, Embase, PubMed Central and Google scholar were searched using the key words "cinnamon" or "cinnamomum" for clinical trials, case reports and case series. Also spontaneous reports about adverse effects of cinnamon were collected from five national and international spontaneous reporting schemes. Thirty eight clinical trials were found, five of them reported adverse events. Twenty case reports and seven case series, as well as, spontaneous reports including 160 adverse events were also included. The most frequent adverse events were gastrointestinal disorders and allergic reactions which were self-limiting in the majority of cases. The available data suggests that despite the safety of cinnamon use as a spice and/or flavoring agent, its use may be associated with significant adverse effects in medicinal uses with larger doses or longer duration of use and should be clinically monitored.