Diabetes mellitus is the sixth leading cause of death in the United States, and most patients with the disease have type 2 diabetes. The effectiveness of cinnamon supplementation in patients with type 2 diabetes has received a great deal of media attention after a study was published in 2003. Although the efficacy of cinnamon in patients with diabetes has not been established, many patients seek other therapies and supplement their prescribed pharmacologic therapy with cinnamon. We conducted a literature search, limited to English-language human studies, using MEDLINE (1966-August 2006), EMBASE (1980-August 2006), International Pharmaceutical Abstracts (1970-August 2006), and Iowa Drug Information Service (1966-August 2006). References from articles and clinical trials were reviewed for additional sources; no abstracts were reviewed. We found two prospective, randomized, double-blind, placebo-controlled, peer-reviewed clinical trials and one prospective, placebo-controlled, peer-reviewed clinical trial that evaluated the efficacy of cinnamon supplementation in patients with type 2 diabetes; a total of 164 patients were involved in these trials. Two of the studies reported modest improvements in lowering blood glucose levels with cinnamon supplementation in small patient samples. One trial showed no significant difference between cinnamon and placebo in lowering blood glucose levels. Overall, cinnamon was well tolerated. These data suggest that cinnamon has a possible modest effect in lowering plasma glucose levels in patients with poorly controlled type 2 diabetes. However, clinicians are strongly urged to refrain from recommending cinnamon supplementation in place of the proven standard of care, which includes lifestyle modifications, oral antidiabetic agents, and insulin therapy.