What are cinnamon’s main drawbacks?

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).^{[1][2][3][4][5][6][7]} 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.^{[8][9][10][11]} 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).^[12][13] Secondly, cinnamon contains coumarin,^{[14][15][16][17]} which can cause liver damage and interfere with blood clotting if ingested in sufficient amounts.^[18][13][19] 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,^{[2][12][13][20]} and cinnamon is considered to be safe for consumption at the levels typically consumed.^[18][13][21]