Some randomized trials find that 1–6 grams of cinnamon a day modestly lowers fasting glucose in people with type 2 diabetes; others find no effect, and effects on A1C are small or absent. The trials use different species, doses and preparations, which makes them hard to compare. Common cassia cinnamon contains coumarin, which can harm the liver at the gram-level doses used in trials; Ceylon cinnamon contains very little. Cinnamon is a reasonable spice, not a treatment.
What cinnamon is
Cinnamon is the dried inner bark of trees in the genus Cinnamomum. Two kinds are sold as food:
- Cassia cinnamon (Cinnamomum cassia, C. burmannii, C. loureiroi) — the common, inexpensive kind, with a strong flavour. It contains substantial amounts of coumarin, a natural compound that can damage the liver in susceptible people at high intakes.
- Ceylon cinnamon (Cinnamomum verum, "true cinnamon") — milder, more expensive, and containing only trace coumarin.
Most trials of cinnamon for blood sugar used cassia, and many did not specify.
How it appears to work
Laboratory studies suggest that compounds in cinnamon — particularly polyphenols such as cinnamaldehyde and procyanidins — may improve insulin signalling, slow the emptying of the stomach and inhibit enzymes that break down carbohydrate. Whether these effects are large enough to matter at dietary doses in humans is the question the trials try to answer.
Evidence by use
| Use | Rating | What the trials show |
|---|---|---|
| Fasting glucose in type 2 diabetes | Mixed | Meta-analyses of 10–20 small trials report a modest reduction in fasting glucose (roughly 10–25 mg/dL) with 1–6 g/day; several individual trials and at least one Cochrane review found no significant effect. Heterogeneity between trials is high. |
| A1C | Mixed | Effects on A1C are smaller and often not statistically significant; most trials were too short to show A1C change reliably. |
| Cholesterol and triglycerides | Limited | Some meta-analyses find small reductions in total cholesterol, LDL and triglycerides; others do not. |
| Prediabetes | Insufficient | A small number of trials, with inconsistent results. |
| Polycystic ovary syndrome | Insufficient | Very few, very small trials. |
The pattern is a modest and unreliable effect on fasting glucose, minimal effect on A1C, and enough disagreement between trials that the U.S. National Center for Complementary and Integrative Health concludes it is not clear whether cinnamon helps with diabetes.
Why the trials disagree
- Different species. Cassia and Ceylon cinnamon differ in composition; many trials did not specify which they used.
- Different preparations. Ground bark, water extracts and alcohol extracts contain different compounds.
- Different doses, from 120 mg to 6 g a day.
- Different participants. Trials in people with poorly controlled diabetes tend to find larger effects than those in well-controlled or non-diabetic people.
- Small size and short duration. Most trials enrolled fewer than 100 people for 6–12 weeks.
Doses studied
Trials used 1–6 grams a day of ground cinnamon (about ½ to 2 teaspoons), or 120–500 mg of an extract, for 4–16 weeks.
Safety: the coumarin problem
Coumarin causes liver damage in sensitive people and has caused liver tumours in animals at high doses. The European Food Safety Authority's tolerable daily intake is 0.1 mg per kilogram of body weight — 7 mg a day for a 70 kg adult. Cassia cinnamon contains roughly 2–7 mg of coumarin per gram, so a trial dose of 3–6 g of cassia a day can exceed the tolerable intake, and 1 g a day approaches it for smaller adults. Ceylon cinnamon contains a fraction of a milligram per gram and is not a concern.
Anyone using cinnamon regularly at supplement-level doses should use Ceylon cinnamon or a coumarin-free extract, and people with liver disease or on medications that affect the liver should avoid high-dose cassia.
Cinnamon can add to the effect of glucose-lowering medications. Allergic reactions and mouth irritation from cinnamon oil are occasionally reported.
Where it appears in supplements
Cinnamon (often as "cinnamon bark extract" or "Cinnulin PF") is a fixture of blood sugar formulas. Two questions to ask of any label: which species, and how much. A formula listing "cinnamon bark 100 mg" contains a small fraction of the amounts studied. See blood sugar supplement research.
Bottom line
Cinnamon is a pleasant spice with a possible, small and inconsistent effect on fasting glucose. Adding it to food is harmless and may help a little. Taking gram-level doses of cassia cinnamon daily is not harmless, and there is no reliable evidence that it improves the outcomes that matter. The broader comparison of ingredients and the guide to foods that affect blood sugar offer more useful levers.
Frequently asked questions
How much cinnamon would I need to affect blood sugar?
Trials that found an effect used 1–6 grams a day — roughly half a teaspoon to two teaspoons of ground cinnamon. That is far more than most people add to food, and at the upper end it exceeds safe coumarin intake for cassia cinnamon.
Which cinnamon is in the supermarket?
In the United States, Canada, the United Kingdom and Australia, "cinnamon" on a supermarket shelf is almost always cassia (Cinnamomum cassia or related species). Ceylon cinnamon (Cinnamomum verum) is usually labelled as such and costs more.
Do cinnamon capsules avoid the coumarin problem?
Only if they are made from Ceylon cinnamon or from a water extract with coumarin removed. Many capsules are simply ground cassia, and their labels rarely state coumarin content.
References
- National Center for Complementary and Integrative Health. Cinnamon (2020). https://www.nccih.nih.gov/health/cinnamon
- National Center for Complementary and Integrative Health (NCCIH). Diabetes and Dietary Supplements: What You Need To Know (2021). https://www.nccih.nih.gov/health/diabetes-and-dietary-supplements-what-you-need-to-know
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