Cinnamon is a spice derived from the bark of trees in the Cinnamomum genus, particularly Cinnamomum zeylanicum (also known as Cinnamomum verum, “Ceylon cinnamon”, or “true cinnamon”) and Cinnamomum cassia (also called “Chinese cinnamon”).^[17][18][19] The bark is typically ground into a powder or used in its stick form. Cinnamon is used for several culinary purposes and is added to several foods, including breakfast cereals, snack foods, breads, cakes, and drinks including tea, coffee, and hot chocolate.^[17][18]

Cinnamon contains several bioactive compounds, including cinnamaldehyde, cinnamic acid, eugenol, and polyphenols.^{[17][18][19][20]} Some of these compounds are claimed to have antioxidant, anti-inflammatory, antimicrobial, and antidiabetic properties.^{[17][18][21][19][20]} Consequently, cinnamon is used in traditional medicine to treat a range of conditions, and, due to its widespread health claims, some people choose to take cinnamon as a dietary supplement.^[17][18][20]

Cinnamon is claimed to treat several conditions, including diabetes and cardiovascular disease, respiratory and digestive disorders, and neurodegenerative conditions like Parkinson’s disease and Alzheimer’s disease.^{[17][18][19][20]} Some in vitro and animal studies have yielded promising results, but clinical evidence is lacking for many of these conditions.

In humans, some meta-analyses of randomized controlled trials have shown that cinnamon can improve glycemic control (reduced blood glucose and HbA1c),^{[1][2][3][4][5]} blood lipids (decreased triglycerides, LDL, total cholesterol),^{[6][7][8][9][10][11]} and blood pressure.^[12][13][14] Supplementation with cinnamon might also improve some biomarkers of inflammation and oxidative stress, including reduced plasma concentrations of interleukin 6 and C-reactive protein.^[15][16][6] Therefore, cinnamon might be useful for the treatment of diabetes and cardiovascular disease. However, many clinical studies have a moderate to high risk of bias,^{[22][23][4][2][6][8][15]} and many of the meta-analyses cited here find large variability (substantial heterogeneity) in the size of the effect between studies. Furthermore, large observational studies show that LDL, HDL, and HbA1c are not different between people who regularly consume cinnamon versus people who do not.^[22] Consequently, more well-controlled randomized clinical trials are needed to definitively establish cinnamon's health benefits in humans.

The main drawback is that the current evidence underpinning the clinical benefits of cinnamon is derived from studies with small sample sizes and low methodological quality (many studies lack a control group, do not blind the participants or investigators from the treatment, and are funded or authored by manufacturers of cinnamon-containing products).^{[22][23][4][2][6][8][15]} Consequently, large high-quality clinical trials are needed to make firm conclusions about cinnamon's health benefits in humans.

A further drawback is that several studies investigating the effect of cinnamon supplements on blood glucose, lipids, and blood pressure (see What are cinnamon’s main benefits?) do not report the participants’ dietary habits (daily energy intake) or physical activity levels (daily energy expenditure). This is problematic, because several such studies also report weight loss in the cinnamon treatment groups, which could be interpreted to mean that cinnamon causes weight loss.^{[24][25][26][27]} However, without information about daily energy intake/expenditure, the direct effects of cinnamon supplementation on changes in body weight are unclear. Furthermore, because weight loss independently improves blood glucose, lipids, and blood pressure, this further complicates the interpretation of cinnamon’s direct effect on such variables and underlines the urgent need for well-controlled clinical trials in this field.

Cinnamon also has some safety concerns. Firstly, ingesting large amounts of cinnamon can cause gastrointestinal issues (diarrhea, nausea, and vomiting).^[28][29] Secondly, cinnamon contains coumarin,^{[30][31][32][33]} which can cause liver damage and interfere with blood clotting if ingested in sufficient amounts.^[34][29][35] For this reason, cinnamon might interact negatively with blood-thinning medications and hepatotoxic drugs and be harmful to people with liver conditions. However, these potential interactions have not been well studied. Because cinnamon can also lower blood glucose and blood pressure (see What are cinnamon’s main benefits?), it is possible that cinnamon could interact with glucose-lowering and blood-pressure-lowering drugs; however, such interactions have not been investigated.

Despite the above-described concerns, adverse reactions caused by ingesting cinnamon are rare and generally mild,^{[23][28][29][36]} and cinnamon is considered to be safe for consumption at the levels typically consumed.^[34][29][37]

Bioactive molecules found in cinnamon affect various physiological processes.^[17][18] For example, cinnamon extracts have been shown to have antimicrobial properties that inhibit bacterial and fungal growth and antioxidant properties that neutralize free radicals to help lower oxidative stress.^[17][18][21] Other in vitro studies have shown that cinnamon extracts and cinnamaldehyde can reduce inflammation by blocking NF-kB activity, reducing cyclooxygenase-2 (COX-2) enzyme activity, blunting nitric oxide (NO) production, and lowering the concentration of proinflammatory cytokines like interleukin 6 (IL-6).^[17][18] These beneficial effects on inflammation might explain how cinnamon supplementation can lower blood glucose, blood pressure, and blood lipids, because conditions in which these variables are elevated are associated with low-grade chronic inflammation. Furthermore, cinnamon’s blood-glucose-lowering capacity might be caused by polyphenols that increase glucose uptake into cells by mimicking the function of insulin.^[17][38][39] However, the precise mechanisms by which cinnamon ingestion may improve certain outcomes are not fully understood and require further exploration.