Probably not by enough to matter: the reviews disagree, the biggest drops come from shaky data, and the American Diabetes Association says not to use cinnamon for blood sugar.
Cinnamon is one of the two supplements people ask me about most. (Apple cider vinegar is the other.) It's cheap, it tastes good, and you've probably seen a headline saying a spoonful a day will fix your A1c.
I went through the research so you don't have to. Here's what it actually says.
The book's grade: UNLIKELY TO HELP. Studies disagree, and the ADA says not to use it for blood sugar.
What the studies found
Cinnamon is one of the most studied spices in diabetes. That sounds good, until you see that the big reviews don't agree with each other.
- A 2023 review of trials found no effect on A1c or fasting sugar (2023 review).
- A 2025 review of 28 trials found A1c down about 0.56%. But the results jumped around so much from study to study that the average is hard to trust (2025 review).
- A 2012 review called cinnamon helpful. Yet the average A1c drop was only 0.09%, which is too small to notice (2012 review).
In the 2025 review, the A1c drop showed up only at 2 grams a day or less, mostly in capsules. Those trials lasted 1 to 4 months.
Why so much disagreement? The trials used different kinds of cinnamon (cassia vs Ceylon), different forms (powder, capsules, water extracts), doses from 1 to 6 grams, and different people. A 2019 review of 16 trials found lower fasting sugar but no change in A1c (2019 review).
When results swing from "nothing" to "a lot" depending on how you pool the trials, the fair summary is simple. Cinnamon is not proven.
The American Diabetes Association looked at all of this too. Its 2026 guidelines say herbs or spices such as cinnamon are "not recommended for glycemic benefits" (American Diabetes Association Standards of Care 2026).
Why it might work (and why that's not proven)
Some researchers think cinnamon helps your cells take in sugar a bit like insulin does. That idea comes mostly from lab and animal studies. It hasn't been shown in people.
One small human test didn't help the case. In 10 people with prediabetes, 6 grams of Ceylon cinnamon did nothing to their sugar after a meal (small study).
So why is cinnamon so popular? Some early small studies made headlines. The larger reviews didn't hold up as well, but the headlines stuck. That's why I grade on the whole body of evidence, not the most exciting study.
Safety
Cinnamon on your food is fine. Capsules are a different story, for a few reasons.
- Your liver. Cassia is the most common cinnamon sold in the U.S. and Canada. It contains coumarin, which might cause or worsen liver disease. Most cassia doesn't have enough to make you sick, but large amounts could harm people with liver disease. The NIH's LiverTox database rates cinnamon a possible rare cause of liver injury. If you have liver disease, avoid large amounts.
- How much. Below 6 grams a day, cinnamon supplements don't appear to pose a health risk, according to the NIH.
- Side effects. Stomach trouble and allergic reactions were the most common, usually mild and short-lived (side-effect review).
- Diabetes medicines. The NIH warns that combining cinnamon supplements with diabetes drugs can cause unwanted side effects. If you take insulin or a sulfonylurea pill, watch for lows.
Buying tip: most store cinnamon is cassia. When researchers tested cinnamon from U.S. stores, cinnamon foods and cinnamon supplements, every sample contained coumarin. Ceylon ("true cinnamon") has only traces (lab testing study). If you take capsules anyway, choose Ceylon.
What I tell my patients
Enjoy cinnamon on your oatmeal. Just don't count on it to lower your A1c, and don't use it in place of your treatment plan.
If you already have cinnamon capsules at home, bring them to your next visit. Ask your doctor or pharmacist: "Are these safe with my liver tests and my diabetes medicines?"
Curious, and have a meter? Eat the same breakfast twice, once with a capsule at the study dose and once without. Check your sugar 1 and 2 hours after you start eating, and repeat both once more. Small differences are likely meter error. On insulin or a sulfonylurea pill, check with your doctor first and keep glucose tablets handy.
Bottom line
Skip cinnamon for blood sugar. It's fine as a spice. But the evidence that it lowers A1c is shaky, the better reviews found little or nothing, and the ADA does not recommend it. The claim that a teaspoon a day will bring your A1c to normal never happened in any trial.
Questions people ask
How much cinnamon do I need to lower my A1c?
There's no proven amount. The 2025 review found an A1c drop only at 2 grams a day or less, mostly in capsules, but its results varied so much that the average is hard to trust. Other reviews found no effect at all.
Is Ceylon cinnamon better than regular cinnamon?
For safety, yes. Ceylon has only traces of coumarin, while cassia, the common store kind, has much more. For blood sugar, neither one is proven. In one small study, 6 grams of Ceylon did nothing to after-meal sugar in people with prediabetes.
Can I take cinnamon with metformin or insulin?
Ask your doctor or pharmacist first. The NIH warns that cinnamon supplements combined with diabetes drugs can cause unwanted side effects. If you take insulin or a sulfonylurea pill, watch for lows.
Want every supplement graded like this?
Cinnamon is one card out of dozens. In Your A1c by the Numbers, I grade every food, habit and supplement on the evidence and give you a real number for each, so you can spend your effort where it counts. Not sure yet? Browse more short reads first.