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Safety File

Cassia Cinnamon for Glucose: The Evidence, the Retraction Concern and the Coumarin Question

Cinnamon is in almost every blood sugar formula, including Gluco Extend. The evidence is weaker than its reputation, and the type of cinnamon matters for safety.

Reviewed October 19, 2026Matched to vendor & ClickBank pages9 cited records
Gluco Extend bottle on a dark pedestal beside the article title: Cassia cinnamon for glucose, and the coumarin question
Figure 1: Safety File: Cassia cinnamon for glucose, and the coumarin question.Clinical blog

Two cinnamons, one important difference

Most cinnamon sold in the U.S. is cassia (Cinnamomum cassia or related species), with a thick bark and a strong flavor. Ceylon cinnamon (C. verum) is thinner, paler and more expensive. Both contain the polyphenols studied for glucose, but cassia also contains much more coumarin, a natural compound that can injure the liver when intake is high and regular[7]. Germany's risk assessment institute set a tolerable daily intake of 0.1 mg of coumarin per kilogram of body weight, and has noted that heavy cassia eaters can exceed it[8].

The glucose evidence

Interest in cinnamon took off after a 2003 trial in which 60 people with type 2 diabetes took 1, 3 or 6 grams of cassia a day for 40 days and saw fasting glucose fall by 18 to 29 percent[4]. In 2025 the journal that published it issued an Expression of Concern about the paper[5], so it is best read alongside later work.

That later work is more modest. A 2013 meta-analysis of ten randomized trials, 543 participants and doses from 120 mg to 6 grams a day found a significant fall in fasting glucose but no significant change in A1C[1]. A Cochrane review found no clear A1C benefit[2], and a 2019 meta-analysis including people with prediabetes found small fasting glucose improvements[3]. In a meal study, 6 grams of cinnamon with rice pudding slowed stomach emptying and lowered the after-meal rise in healthy volunteers[6]. NCCIH's summary is that cinnamon has not been shown to help any health condition[9].

What it means for a supplement

The doses that showed anything were grams, not milligrams. A capsule that holds cinnamon alongside nine other ingredients carries a small fraction of a gram at most. The upside of a small dose is that coumarin exposure from it is also small: well under the tolerable intake for an adult unless the extract is concentrated or you eat a lot of cassia as well. If your goal is cinnamon itself, a labeled cinnamon product (ideally stating whether it is Ceylon or cassia, and its coumarin content) is easier to evaluate.

Who should be careful

People with liver disease, anyone taking several medicines processed by the liver, and people on warfarin should mention cassia supplements to their clinician. People on glucose-lowering drugs should treat cinnamon as a possible addition to their effect, even if modest.

Quick answers

Does cinnamon lower A1C?

Meta-analyses found lower fasting glucose but no reliable change in A1C, the three-month marker clinicians use.

Is cassia cinnamon bad for the liver?

Only at high, regular intake, because of its coumarin. Small amounts in food or a shared capsule are well below the tolerable daily intake for most adults.

Records cited on this page

Every link opens the original record. The trials tested single botanicals at stated doses and preparations; none tested the Gluco Extend capsule, whose amounts are not published.

  1. 1Allen RW, Schwartzman E, Baker WL, Coleman CI, Phung OJ. Cinnamon use in type 2 diabetes: an updated systematic review and meta-analysis. Ann Fam Med. 2013;11(5):452-459. pubmed.ncbi.nlm.nih.gov/24019277/
  2. 2Leach MJ, Kumar S. Cinnamon for diabetes mellitus. Cochrane Database Syst Rev. 2012;(9):CD007170. pubmed.ncbi.nlm.nih.gov/22972104/
  3. 3Deyno S, Eneyew K, Seyfe S, et al. Efficacy and safety of cinnamon in type 2 diabetes mellitus and pre-diabetes patients: a meta-analysis and meta-regression. Diabetes Res Clin Pract. 2019;156:107815. pubmed.ncbi.nlm.nih.gov/31425768/
  4. 4Khan A, Safdar M, Ali Khan MM, Khattak KN, Anderson RA. Cinnamon improves glucose and lipids of people with type 2 diabetes. Diabetes Care. 2003;26(12):3215-3218. pubmed.ncbi.nlm.nih.gov/14633804/
  5. 5American Diabetes Association. Expression of Concern: Khan A, et al. Cinnamon improves glucose and lipids of people with type 2 diabetes. Diabetes Care 2003;26:3215-3218. Diabetes Care. 2025;48(9):1646. pubmed.ncbi.nlm.nih.gov/40834253/
  6. 6Hlebowicz J, Darwiche G, Björgell O, Almér LO. Effect of cinnamon on postprandial blood glucose, gastric emptying, and satiety in healthy subjects. Am J Clin Nutr. 2007;85(6):1552-1556. pubmed.ncbi.nlm.nih.gov/17556692/
  7. 7Abraham K, Wöhrlin F, Lindtner O, Heinemeyer G, Lampen A. Toxicology and risk assessment of coumarin: focus on human data. Mol Nutr Food Res. 2010;54(2):228-239. pubmed.ncbi.nlm.nih.gov/20024932/
  8. 8German Federal Institute for Risk Assessment (BfR). Frequently asked questions about coumarin in cinnamon and other foods. www.bfr.bund.de/en/frequently_asked_questions_about_coumarin_in_cinnamon_and_other_foods-8487.html
  9. 9National Center for Complementary and Integrative Health. Cinnamon. www.nccih.nih.gov/health/cinnamon

Give Gluco Extend a measured, six-month trial

ClickBank processes the order. Write down a few fasting and after-dinner readings before the first capsule, keep the walk after meals and the vegetables-first plate, save the receipt email, and if your numbers and your doctor see no change by the fifth month, send the bottles back while the window is still open.

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