Cinnamon is one of the most popular supplements for blood sugar, and the idea comes from a real body of research: dozens of small trials and more than a dozen meta-analyses, studies that pool the results of many trials. But the picture is messier than the headlines. The trial that made cinnamon famous now carries a warning from the journal that published it. Most studies were short, and many did not say which kind of cinnamon they used. And the kind most people buy, cassia, contains enough coumarin, a natural compound that can harm the liver in sensitive people, to be a concern at supplement doses.
This guide covers what the trials actually found for fasting glucose and HbA1c, how cassia and Ceylon cinnamon differ, how much cassia is too much, what is known about safety and drug interactions, and how to read a cinnamon supplement label. For the wider picture, see our blood sugar supplements guide.
The short version
- The effect is small and inconsistent. Pooled trials in type 2 diabetes found cinnamon lowered fasting glucose a little on average, but results for HbA1c (a measure of average blood sugar over about 3 months) are split, and most trials lasted 4 to 16 weeks.
- The famous study is under a cloud. In 2025 the journal that published the 2003 trial behind the "cinnamon cuts blood sugar by up to 29 percent" claim issued an expression of concern about it, a formal notice that questions the reliability of a published study.
- Diabetes guidelines do not recommend it. The American Diabetes Association's 2026 Standards of Care say supplements of herbs and spices such as cinnamon are not recommended for blood sugar benefits.
- Cassia vs Ceylon: most cinnamon sold in North America is cassia, which can be high in coumarin. Ceylon has only traces. We found no trial that compared the two for blood sugar.
- How much cassia is too much: Germany's risk assessment agency says a 60 kilogram adult reaches Europe's safe daily coumarin limit with about 2 grams of average cassia a day (about three-quarters of a teaspoon), and with about a third of that if the cassia is high in coumarin.
- If you try it, choose Ceylon for daily use, keep taking your prescribed medicines, and check your blood sugar if you take diabetes medicines.
Cassia vs Ceylon at a glance
| Cassia | Ceylon | |
|---|---|---|
| Species | C. cassia, burmannii, loureiroi (Saigon) | C. verum (C. zeylanicum) |
| Grown in | China, Indonesia, Vietnam | Mainly Sri Lanka |
| In US stores | Most common | Less common; look for "Ceylon" on the label |
| Coumarin | About 3 mg per gram on average, up to about 10 mg per gram | Traces |
| Sticks | One thick, hard layer | Thin layers rolled like a cigar |
| Blood sugar trials | Many older trials, mixed results | A few small trials, mostly since 2022; small effects |
| For daily supplement doses | Coumarin can exceed the safe limit | Better choice |
Coumarin figures are from Germany's Federal Institute for Risk Assessment (BfR). Many older trials did not say which species they used.
Ceylon vs cassia: which cinnamon is in your jar?
"Cinnamon" covers several related trees. Ceylon cinnamon (Cinnamomum verum, also called C. zeylanicum), often called "true" cinnamon, grows mainly in Sri Lanka. The cassia group includes Chinese cassia (C. cassia, also called C. aromaticum), Indonesian cassia (C. burmannii) and Saigon or Vietnamese cinnamon (C. loureiroi). US rules list all of them as spices, and even list some cassia species under the name "cinnamon," so a label that just says cinnamon may be cassia.
The US National Center for Complementary and Integrative Health (NCCIH) says cassia is the most common type sold in North America. A 2013 University of Mississippi study found that all the cinnamon it bought locally, including cinnamon in foods and supplements, was probably Indonesian cassia, and that the cassia species, especially Saigon and Indonesian cassia, contained substantial coumarin while Ceylon had only traces.
Telling them apart is hard once the spice is ground. Germany's BfR says that in powder form consumers can hardly tell them apart. Sticks are easier: cassia forms a thick, single-layered roll, while Ceylon looks like a cut cigar made of many thin layers. Mislabeling also happens. In a 2025 European Commission survey of 104 retail cinnamon samples, about two-thirds failed quality or safety checks or were suspected of fraud, and some samples labeled Ceylon were actually cassia.
Does cinnamon lower blood sugar and A1C? What the trials found
Most of the research is in people with type 2 diabetes, often alongside their usual medicines. Several meta-analyses have looked at it, with different conclusions:
- Fasting glucose: most pooled analyses found a small average drop. A 2013 analysis of 10 trials found fasting glucose about 25 mg/dL lower (mg/dL, milligrams per deciliter, is the US unit for blood sugar; a fasting level under 100 mg/dL is considered normal), and a 2025 analysis of 28 trials with about 3,000 people found about 15 mg/dL lower. But the trials varied enormously in their results, and a 2012 Cochrane review, from an independent network known for strict methods, called the fasting glucose evidence inconclusive.
- HbA1c: the results are split. The 2013 analysis, the Cochrane review and a 2019 analysis of 16 trials found no significant change. A 2012 analysis found a reduction of 0.09 percentage points, and the 2025 analysis found about 0.56 points.
- Insulin resistance (when the body responds less well to insulin): several analyses found modest improvements. But a 12-week Swedish trial in 21 people with impaired glucose tolerance (blood sugar above normal after a sugar drink, but below the diabetes range) measured insulin sensitivity directly and found no effect, even at 12 grams a day.
Individual trials point both ways. In a 4-month German trial of 79 people with type 2 diabetes, a cinnamon extract lowered fasting glucose 10.3 percent versus 3.4 percent on placebo, with no difference in HbA1c. In a 90-day Brazilian trial of 160 people taking oral diabetes medicines, 3 grams a day lowered HbA1c by 0.2 percentage points and fasting glucose by about 10 mg/dL compared with placebo, which the authors called modest. Trials in the Netherlands, Thailand and Iran found no difference from placebo.
How strong is the evidence?
Not very, for several reasons:
- Short trials: most lasted 4 to 16 weeks, and in PubMed searches through November 2026 we found no published blood sugar trial of cinnamon alone in diabetes lasting longer than 4 months.
- No long-term health outcomes: no trial we found has measured diabetes complications, heart disease or deaths.
- Unclear products: doses ranged from 120 milligrams of extract to 6 grams of powder (12 grams in one trial), and many trials did not say which species they used. A US Pharmacopeia review warned that results from one cinnamon product cannot be assumed to apply to another.
- Quality and funding: the Cochrane review rated the risk of bias (the chance that flaws in how a trial was run skewed its results) high or unclear in all but two of its 10 trials, and some trials were funded by, or linked to, companies that make cinnamon products, or run by researchers who declared a patent.
- The longest trial showed no benefit: a 12-month US military trial gave 229 people with prediabetes either a water extract of cinnamon (Cinnulin PF, 1 gram a day) or placebo, on top of an intensive lifestyle program. In results posted to the trial registry, diabetes developed in 15 of 47 people who finished on cinnamon and 11 of 43 on placebo (about 32 versus 26 percent). More than half the participants dropped out, no statistical analysis was posted, and as far as we can find the results were never published in a journal.
That is why the American Diabetes Association's 2026 Standards of Care say supplements of herbs or spices such as cinnamon are "not recommended for glycemic benefits," meaning blood sugar benefits (the same Standards suggest cinnamon as a spice to season food instead of salt). It is also why the NCCIH says it is unclear whether cinnamon helps with diabetes. Cinnamon is not a substitute for diabetes medicines, and nothing here suggests it can treat or prevent diabetes.
Does Ceylon work as well as cassia?
In PubMed searches through November 2026, we found no trial that compared cassia and Ceylon head to head. Most older positive trials either used cassia or did not say. A small 2012 Swedish study found that 6 grams of Ceylon taken with a glucose drink did not change the blood sugar response in 10 people with impaired glucose tolerance. Its authors suggested switching to Ceylon might lose cassia's effect.
Newer Ceylon trials suggest Ceylon may have small effects too. The 90-day Brazilian trial above used Ceylon, according to its published protocol, and found the 0.2 point HbA1c drop. A 4-month Sri Lankan trial of a Ceylon extract in 210 people with type 2 diabetes reported lower fasting glucose and HbA1c compared with the start of the study. Its authors declared holding a patent (the trial tested an extract they developed), and the abstract does not report the difference from placebo. A 12-week Sri Lankan trial in 150 adults with high LDL cholesterol, funded by a spice company, found a Ceylon extract lowered fasting blood sugar slightly. It did not lower LDL cholesterol, the main outcome the trial was designed to test.
The honest summary: we found no evidence that one species lowers blood sugar more than the other, and coumarin is a good reason to prefer Ceylon for daily, supplement-sized amounts.
The coumarin problem: how much cassia cinnamon a day is too much?
Coumarin is a natural flavor compound found in cassia, tonka beans and some other plants. At doses well above what food usually supplies, it can harm the liver in a small group of sensitive people; the BfR notes that when coumarin was used as a medicine, even relatively low doses taken for a few weeks caused liver damage in some of them. When coumarin was used as a drug for limb swelling (lymphedema) at about 400 milligrams a day, roughly 65 times the adult limit described below, a few patients developed liver inflammation. In large patient series, about 2 to 4 in 1,000 developed liver problems, and in one 6-month trial, 6 percent of women had blood test signs of liver toxicity. Why some people are sensitive is not known, and there is no known test to identify them.
The limit. Europe's food safety agency (EFSA) set a tolerable daily intake of 0.1 milligrams of coumarin per kilogram of body weight, the amount considered safe to eat every day for a lifetime. Germany's BfR arrived at the same value and says it also covers especially sensitive people. That is 6 milligrams a day for a 60 kilogram (132 pound) adult and 1.5 milligrams for a 15 kilogram child.
How much cassia that is. The BfR says cassia averages about 3,000 milligrams of coumarin per kilogram (3 milligrams per gram), with the highest levels around 10,000 (10 milligrams per gram). Ceylon contains only traces. Using a teaspoon of ground cinnamon at 2.6 grams (USDA):
| Daily cassia | Coumarin (average cassia) | Share of limit, 60 kg adult |
|---|---|---|
| 1 g (two 500 mg capsules) | About 3 mg | About half |
| 2 g (about 3/4 teaspoon) | About 6 mg | At the limit |
| 2.6 g (1 teaspoon) | About 8 mg | About 1.3 times |
| 3 g (common trial dose) | About 9 mg | 1.5 times |
| 6 g | About 18 mg | 3 times |
Our arithmetic from BfR coumarin averages. With high-coumarin cassia, multiply by about 3: a 60 kilogram adult would reach the limit at about 0.6 grams, roughly a quarter teaspoon. A 15 kilogram child reaches the limit at about 0.5 grams of average cassia.
Keep it in perspective. The limit is a lifetime daily benchmark. The BfR says slightly exceeding it for one to two weeks is safe, and that a health risk is possible only if people eat very large amounts of high-coumarin cassia over a long period. Liver injury blamed on cinnamon itself is rare: the NIH's LiverTox database describes only isolated reports. The concern is daily cassia supplements and heavy daily use, not an occasional sprinkle on oatmeal or a holiday cookie. Small children reach their limit much sooner (about 0.5 grams of average cassia a day for a 15 kilogram child), so go easy on cassia in foods they eat every day.
Extracts and tea. Some water extracts of cassia are marketed as low in coumarin, but coumarin does dissolve in hot water: in a German study, coumarin from cinnamon tea was absorbed at least as well as from capsules. None of the supplement labels we reviewed in the NIH Dietary Supplement Label Database stated coumarin content, and while US rules ban coumarin added to food as such, we found no US limit for the coumarin that occurs naturally in cinnamon or cinnamon supplements.
Prediabetes, weight, cholesterol, PCOS and how it might work
- Prediabetes: in a 12-week trial of 54 people with prediabetes, fasting glucose rose on placebo but stayed level on cinnamon, a difference of about 5 mg/dL. A crossover study of 18 people with obesity and prediabetes, who took 4 grams a day for 4 weeks, found lower 24-hour glucose than during 4 weeks on placebo, measured with continuous glucose monitors (wearable sensors). The 12-month prevention trial above found no fewer diabetes cases.
- Weight: pooled trials found about 1 kilogram (2 pounds) more weight loss, and the NCCIH says it is unclear whether cinnamon helps.
- Cholesterol and blood pressure: some meta-analyses found lower triglycerides and LDL cholesterol, and small blood pressure drops, but results are inconsistent and the trials were short.
- PCOS: a few small trials in polycystic ovary syndrome (PCOS, a hormonal condition often linked to insulin resistance) reported lower fasting insulin or more regular cycles; one found more regular cycles but no change in insulin resistance.
- Brain and Alzheimer's: the evidence is from animals and lab studies only; in PubMed searches through November 2026 we found no human trial.
How it might work. How cinnamon might affect blood sugar is not settled. One small study in healthy adults found that 6 grams slowed stomach emptying after a meal (a follow-up at 1 or 3 grams found no such effect), and lab studies suggest some compounds mimic insulin in cells. In a 2024 study of 19 people with type 2 diabetes, 3 or 6 grams of cinnamon powder stirred into water blunted the rise in blood sugar after a meal, while the same amounts in capsules had little or no effect. That was a single-meal test, so it says nothing about long-term control.
Cinnamon side effects and drug interactions
- Diabetes medicines (metformin, insulin and others): in theory, cinnamon could add to the effect of metformin, insulin or other glucose-lowering drugs. We found no case reports of cinnamon causing low blood sugar, and a trial in teenagers with type 1 diabetes found no extra low blood sugar episodes. If you take diabetes medicines, check your glucose more often when you start, and tell your doctor.
- Warfarin: coumarin is often confused with warfarin (Coumadin), but plain coumarin is not a blood thinner; warfarin is a different, related molecule. We found no good studies of cinnamon with warfarin, so if you take it, keep any supplement steady and tell your anticoagulation clinic. See our guide to supplement and drug interactions.
- Liver: people with liver disease, and anyone taking medicines that affect the liver, should avoid high-dose cassia. One case report described hepatitis (liver inflammation) in a 73-year-old woman about a week after she started cinnamon supplements while taking a statin.
- Mouth and skin reactions: cinnamaldehyde, the compound that gives cinnamon its flavor, can cause allergic mouth sores from cinnamon gum, candy and toothpaste.
- The cinnamon challenge: never try to swallow a spoonful of dry cinnamon; inhaling the powder can injure the lungs.
- Pregnancy and breastfeeding: the NCCIH says food amounts appear safe in pregnancy but larger amounts of Ceylon are considered unsafe, and little is known about larger amounts while breastfeeding.
- Lead: in 2024 the FDA warned about elevated lead in some ground cinnamon products sold at discount retailers, after a 2023 recall of cinnamon applesauce pouches linked to lead poisoning in children.
How to choose a cinnamon supplement
- Check the species. "Ceylon," "Cinnamomum verum" or "zeylanicum" means Ceylon. "Cassia," "C. cassia," "aromaticum," "burmannii," "loureiroi" or "Saigon" means cassia. "Cinnamomum spp." or "and/or" means it may be cassia. Many labels just say "cinnamon"; if no species is named, assume it is probably cassia.
- Prefer Ceylon for daily use. The cassia capsule labels we reviewed suggest about 1 gram a day. At average coumarin levels that is about half the limit for a 60 kilogram adult; with high-coumarin cassia it can be up to about 1.7 times the limit.
- Treat "equivalent to" claims carefully. Extracts often say they are "equivalent to" several grams of cinnamon; that is a manufacturer ratio, not a measured effect.
- Check "low coumarin" claims. If a cassia extract is sold as low in coumarin, look for a stated coumarin amount per serving, or ask the company for a lab test result. Hot water does extract coumarin, so "water extract" alone does not guarantee it.
- Look for third-party testing. Given the lead findings above, a third-party testing seal (such as USP or NSF) shows the product was independently tested, including for contaminants, which adds some assurance.
- Doses studied: most trials used 1 to 6 grams of powder (one used 12 grams) or 120 milligrams to 1 gram of extract a day. No dose has been shown to work reliably.
- Do not replace medicines. Use cinnamon as a flavor or an add-on you discuss with your doctor, not instead of prescribed treatment. If you are comparing options, see our guides to curcumin for blood sugar and berberine supplements.
Frequently asked questions
Does cinnamon lower blood sugar?
Slightly, on average, in some studies. Pooled trials in type 2 diabetes found fasting glucose a little lower, but results for HbA1c (average blood sugar over about 3 months) are split, most trials lasted 4 to 16 weeks, and the American Diabetes Association does not recommend cinnamon supplements for blood sugar. It is not a substitute for diabetes medicine.
Does cinnamon lower A1C?
The evidence is mixed. A1C (also called HbA1c) reflects average blood sugar over about 3 months. Some meta-analyses, which pool many trials, found small reductions, from about 0.09 to 0.56 percentage points, while others, including a Cochrane review, found no significant change. A 90-day trial of Ceylon cinnamon found a 0.2 point drop compared with placebo, a modest change. The American Diabetes Association does not recommend cinnamon supplements for blood sugar.
How much cinnamon should I take a day for blood sugar?
No dose has been shown to work reliably. Most trials used 1 to 6 grams of powder (one used 12 grams) or 120 milligrams to 1 gram of extract a day. If you use cassia, Germany's risk assessment agency says about 2 grams a day of average cassia brings a 60 kilogram adult to Europe's safe daily coumarin limit, so Ceylon is the better choice for daily use.
Is Ceylon cinnamon better than cassia for diabetes?
We found no trial that compared them head to head. Most older trials used cassia or did not say, and a few newer Ceylon trials found effects of a similar small size. Ceylon has only traces of coumarin, so it is the safer choice for daily, supplement-sized amounts.
How much cassia cinnamon is safe per day?
Cassia contains coumarin, a natural compound that can harm the liver in sensitive people at doses well above what food usually supplies. Europe's tolerable daily intake for coumarin, the amount considered safe every day for a lifetime, is 0.1 milligrams per kilogram of body weight. Germany's risk assessment agency says a 60 kilogram adult reaches it with about 2 grams of average cassia a day (about three-quarters of a teaspoon) and a 15 kilogram child with about 0.5 grams. High-coumarin cassia reaches the limit at about a third of those amounts. The agency says slightly exceeding it for a week or two is safe.
How can I tell if my cinnamon is Ceylon or cassia?
Check the label for the species: Ceylon, Cinnamomum verum or zeylanicum means Ceylon; cassia, aromaticum, burmannii, loureiroi or Saigon means cassia. Ceylon sticks look like a cigar made of many thin layers, while cassia sticks are a single thick roll. Ground cinnamon is very hard to tell apart, and mislabeling happens. If a label just says cinnamon, assume it is probably cassia.
What kind of cinnamon is sold in US grocery stores?
Mostly cassia. The NIH's complementary health center says cassia is the most common type sold in North America, and a 2013 study found the cinnamon it bought in US stores was probably Indonesian cassia. Ceylon is less common and is usually labeled as Ceylon.
Can cinnamon damage your liver?
Rarely. Coumarin, a natural compound in cassia, can harm the liver in a small group of sensitive people at doses well above what food usually supplies, which is why Europe set a daily limit. Liver injury blamed on cinnamon itself is limited to isolated case reports. People with liver disease should avoid high-dose cassia supplements.
Can I take cinnamon with metformin or insulin?
Talk with your doctor first. Cinnamon could add to the effect of glucose-lowering medicines in theory, though we found no case reports of it causing low blood sugar. If you start it, check your blood sugar more often, and do not reduce your medicines on your own.
Does cinnamon interact with warfarin?
Coumarin, the natural compound in cassia, is often confused with warfarin (sold as Coumadin), but coumarin itself is not a blood thinner; warfarin is a different, related molecule. We found no good studies of cinnamon with warfarin, so if you take it, keep any supplement use steady and tell your anticoagulation clinic.
Can cinnamon help with prediabetes?
The evidence is weak. Short trials found small effects, such as keeping fasting glucose from rising over 12 weeks. A 12-month trial of a cinnamon extract in people with prediabetes posted results showing no fewer diabetes cases than placebo, though many participants dropped out. Lifestyle changes such as diet, activity and weight loss are the approaches with strong evidence.
Does cinnamon help you lose weight?
Not much. Pooled trials found about 1 kilogram (2 pounds) more weight loss on average, and the NIH's complementary health center says it is unclear whether cinnamon helps with weight.
Is cinnamon tea or cinnamon water good for blood sugar?
We found no long-term trials of cinnamon tea for blood sugar. In one small meal study in people with type 2 diabetes, cinnamon powder stirred into water blunted the rise in blood sugar after a meal, and a small study in healthy adults found that cassia tea slightly lowered the rise after a glucose drink. Hot water also extracts coumarin well, so cassia tea counts toward the coumarin limit.
Do cinnamon capsules work as well as cinnamon spice?
We found no trial that compared them for long-term blood sugar control. Many trials gave cinnamon in capsules, most at 1 to 6 grams of powder (one used 12 grams) or 120 milligrams to 1 gram of extract a day. In one 2024 meal study of 19 people with type 2 diabetes, 3 or 6 grams of cinnamon powder stirred into water blunted the rise in blood sugar after a meal, while the same amounts in capsules had little or no effect. That was a single-meal test, so it does not show that capsules fail over months.
What should I look for in a cinnamon supplement?
Check the label for the species. Ceylon (Cinnamomum verum or zeylanicum) is the better choice for daily use because it has only traces of coumarin. If the label lists cassia, Saigon cinnamon, Cinnamomum spp. or just says cinnamon, assume it may be high in coumarin. Treat "equivalent to" claims on extracts as manufacturer ratios, not proof of effect. No dose has been shown to work reliably, and cinnamon should not replace prescribed diabetes medicines.
Is cinnamon safe during pregnancy?
The NIH's complementary health center says cinnamon in food amounts appears safe in pregnancy, but larger amounts of Ceylon are considered unsafe and little is known about larger amounts while breastfeeding. Avoid cinnamon supplements unless your provider recommends them.
The bottom line
Cinnamon may lower fasting blood sugar slightly in people with type 2 diabetes, but the trials are short, inconsistent and of mixed quality, the most famous one is under an expression of concern, and the American Diabetes Association does not recommend cinnamon supplements for blood sugar. It is not a diabetes treatment.
The clearer issue is safety: most supermarket cinnamon is cassia, and daily supplement doses of cassia can exceed Europe's coumarin limit. If you want to use cinnamon regularly, choose Ceylon, keep your medicines as prescribed, and check your blood sugar if you take diabetes medicines.