Green Tea
EVIDENCE: MIXEDAlso sold as: Camellia sinensis, green tea extract, EGCG, catechins
Green tea's catechins, especially EGCG, have been tested for blood sugar and weight. Meta-analyses found small drops in fasting glucose and HbA1c and little effect on weight. Concentrated extracts can rarely injure the liver at high EGCG doses.
Compiled by iSupplementReviews editorial team ·Researched 2026-10-16 · 4 sources
- Type
- Botanical leaf extract
- Studied dose
- Catechins roughly 300–800 mg/day in glucose and weight trials
- Best form
- extract with stated EGCG/catechin and caffeine content, taken with food
- Upper limit
- EFSA flagged liver risk at ≥800 mg EGCG/day from supplements
- Our rating
- MIXED — Some human evidence, inconsistent or indirect
What is green tea?
Green tea is unfermented tea leaf (Camellia sinensis), and its extracts concentrate catechins, mainly EGCG, along with caffeine. It is one of the most studied plant foods.
Other names and forms
Green tea extract, EGCG, decaffeinated green tea extract. Extracts differ widely in catechin and caffeine content.
Where does green tea come from?
Steamed or pan-fired tea leaves, mostly from China and Japan.
How does green tea work in the body?
Catechins and caffeine may modestly raise energy use and fat burning, and catechins may slow carbohydrate absorption. Effects in people are small.
What is green tea claimed to do?
Improve blood flow, lower blood sugar, support weight loss and provide antioxidants.
What does the research show?
| Claim | Evidence | Dose studied | Verdict |
|---|---|---|---|
| Fasting glucose and HbA1c | Meta-analyses of RCTs | Varied, often several hundred mg catechins | Small positive |
| Weight loss | Cochrane review | Varied | Very small |
| Blood flow (any organ) | Small vascular studies | Varied | Weak |
Blood sugar
Pooled trials show small reductions in fasting glucose and HbA1c. Liu et al. (2013) analyzed 17 randomized trials. Zheng et al. (2013) found catechins lowered fasting glucose, with little effect on HbA1c or insulin.
Weight
Green tea preparations produced only a small weight loss, which reviewers judged not clinically important. That was the conclusion of the Jurgens Cochrane review (2012).
What dose was studied?
| Goal | Dose | Duration |
|---|---|---|
| Glucose markers | Catechins ~300–800 mg/day | 8–24 weeks |
| Weight | Catechins with or without caffeine | 12+ weeks |
Side effects
Caffeine-related effects; rare liver injury with concentrated extracts.
Drug interactions
- Nadolol and some other drugs: green tea can lower absorption (moderate).
- Stimulants: caffeine adds up (moderate).
- Warfarin: large amounts of green tea may interfere (minor).
Who should not take green tea extract?
People with liver disease; anyone taking high-dose EGCG on an empty stomach should reconsider (Oketch-Rabah et al., 2020).
What the authorities say
The U.S. Pharmacopeia added a liver-risk caution for green tea extract products, and EFSA flagged EGCG intakes of 800 mg a day or more from supplements. Brewed tea is considered safe.
What to know about green tea
Green tea extract may slightly lower fasting glucose and HbA1c, with very little effect on weight. Studied amounts are usually hundreds of milligrams of catechins.
Frequently asked questions about Green Tea
Does green tea lower blood sugar?
Slightly. A 2013 meta-analysis of 17 randomized trials found small reductions in fasting glucose and HbA1c with green tea or its extract, and the effect was clearer in some subgroups.
Does green tea extract help weight loss?
Very little. A Cochrane review found green tea preparations produced a small, likely not clinically important, weight loss in overweight adults.
Can green tea extract harm the liver?
Rarely, yes. A U.S. Pharmacopeia review linked concentrated extracts, especially high EGCG doses taken on an empty stomach, to cases of liver injury. Brewed tea is not the same risk.
References
- [1]Liu K et al. — Effect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomized controlled trials (Am J Clin Nutr, 2013)
- [2]Jurgens TM et al. — Green tea for weight loss and weight maintenance in overweight or obese adults (Cochrane Database Syst Rev, 2012)
- [3]Oketch-Rabah HA et al. — United States Pharmacopeia (USP) comprehensive review of the hepatotoxicity of green tea extracts (Toxicol Rep, 2020)
- [4]Zheng XX et al. — Effects of green tea catechins with or without caffeine on glycemic control in adults: a meta-analysis of randomized controlled trials (Am J Clin Nutr, 2013)
Products we reviewed containing Green Tea
Based on recorded labels or seller-described formulas. Inclusion does not establish a match to the strains, preparations or amounts studied here; check each review's disclosure limits.