Green Tea

Metadata

Approved Indications

Commission E (Germany)

• Approved: Mental and physical fatigue (as a caffeine-containing stimulant herb)
• The Commission E monograph for green tea acknowledges its traditional use and the pharmacological activity of its caffeine and polyphenol constituents
• Green tea is included in the Expanded Commission E Monographs (Blumenthal et al.)

ESCOP

• No dedicated ESCOP monograph for Camellia sinensis non fermentatum folium has been published
• Green tea catechins are referenced in broader discussions of polyphenol-containing herbs but lack a standalone ESCOP therapeutic monograph

EMA/HMPC

• Traditional use registration (Article 16d, Directive 2001/83/EC): EMA/HMPC/283630/2012 (finalized November 2013)
• Indication: Traditional herbal medicinal product used to relieve symptoms of fatigue and sensation of weakness
• Preparations covered: Comminuted herbal substance, powdered herbal substance, and herbal preparations in solid dosage forms for oral use
• The EMA confirmed that the requirements of at least 30 years in medicinal use and at least 15 years of use in the EU were fulfilled
• Not granted well-established use status for cardiovascular or metabolic indications

Agreement/Disagreement Analysis

• Commission E and EMA/HMPC agree on the fatigue/asthenia indication based on the caffeine and L-theanine content
• No European regulatory body has granted well-established use status for cardiovascular, metabolic, or cancer-preventive indications despite extensive observational and clinical trial data
• EFSA (2011) rejected health claims for green tea catechins related to maintenance of normal blood LDL-cholesterol, blood glucose, blood pressure, and body weight, citing insufficient evidence of a cause-and-effect relationship
• EFSA (2018) specifically assessed the safety of green tea catechins, concluding that EGCG doses at or above 800 mg/day from supplements may cause hepatotoxicity
• The disconnect between the large body of clinical research and the conservative regulatory position reflects the heterogeneity of preparations and the modest effect sizes observed in meta-analyses

Conditions Treated

Primary (Evidence from Meta-Analyses of RCTs)

• Hyperlipidemia (modest LDL and total cholesterol reduction) — Evidence Level: B
• Mild hypertension (modest systolic blood pressure reduction) — Evidence Level: B
• Overweight/obesity (small but significant effects on body weight and BMI) — Evidence Level: B
• Mental fatigue and reduced alertness (caffeine + L-theanine synergy) — Evidence Level: B

Secondary (Emerging or Mixed Evidence)

• Type 2 diabetes / insulin resistance — Mixed results; some RCTs show improved HOMA-IR but meta-analyses overall are inconclusive — Evidence Level: C
• Cancer risk reduction — Observational meta-analyses suggest 9% overall risk reduction; individual RCTs are mixed — Evidence Level: C
• Cognitive function / mild cognitive impairment — L-theanine and caffeine combination shows promise; limited RCT data — Evidence Level: C
• Endothelial function and cardiovascular protection — Mechanistic and observational evidence is strong; RCT data is limited — Evidence Level: C
• Antioxidant status and oxidative stress reduction — Evidence Level: C

Traditional Use

• Mental and physical fatigue (Commission E, EMA/HMPC)
• Diuretic (Traditional Chinese Medicine)
• Astringent for mild diarrhea (traditional use)
• Digestive aid (traditional use across multiple cultures)

Mechanism of Action

Active Compounds

Green tea contains a complex mixture of bioactive constituents. The principal catechin, (-)-epigallocatechin-3-gallate (EGCG), accounts for 50-60% of total catechins and is the most pharmacologically active compound. Other relevant catechins include (-)-epicatechin (EC), (-)-epicatechin-3-gallate (ECG), and (-)-epigallocatechin (EGC). Non-catechin bioactives include caffeine (2.5-4.2%), L-theanine (an amino acid unique to tea), theaflavins, and flavonols (quercetin, kaempferol glycosides).

1. Antioxidant and Redox Modulation

• EGCG and other catechins are potent free radical scavengers with activity against superoxide, hydroxyl radicals, and peroxyl radicals
• Upregulation of endogenous antioxidant defenses via the Nrf2/HO-1 pathway through activation of p38 MAPK and ERK1/2 signaling
• Inhibition of pro-oxidant enzymes including xanthine oxidase and NADPH oxidase
• Protection of LDL from oxidation (anti-atherogenic mechanism)

2. Endothelial Function and Vasodilation

• EGCG activates endothelial nitric oxide synthase (eNOS) through a PI3K/Akt-dependent pathway, increasing nitric oxide (NO) bioavailability [PubMed: 14645258]
• Improvement of endothelium-dependent vasodilation
• Reduction of endothelin-1 expression
• Protection of endothelial cells from oxidative damage

3. Anti-Inflammatory

• Suppression of NF-kB activation in macrophages and endothelial cells
• Inhibition of iNOS expression in lipopolysaccharide-activated immune cells
• Downregulation of pro-inflammatory cytokines (TNF-alpha, IL-1beta, IL-6)
• Inhibition of adhesion molecule expression (VCAM-1, ICAM-1) in coronary artery endothelial cells
• COX-2 inhibition

4. Lipid Metabolism

• Inhibition of intestinal lipid absorption through interaction with bile acid metabolism
• Upregulation of LDL receptor expression in hepatocytes
• Inhibition of squalene epoxidase and HMG-CoA reductase (weak compared to statins)
• Enhancement of fecal fat excretion
• Modulation of lipogenesis gene expression

5. Metabolic and Thermogenic Effects

• Caffeine and catechins synergistically increase thermogenesis and fat oxidation
• Inhibition of catechol-O-methyltransferase (COMT), prolonging norepinephrine activity
• Phosphodiesterase inhibition by caffeine enhances cAMP levels
• Modest enhancement of basal metabolic rate (4-5%)

6. Cognitive and Neurological

• L-theanine: Crosses the blood-brain barrier; increases alpha brain wave activity; promotes relaxation without sedation; modulates serotonin, dopamine, and GABA neurotransmission
• Caffeine: Adenosine receptor antagonist; enhances alertness, attention, and reaction time
• L-theanine + caffeine synergy: The combination improves attention and task-switching accuracy while L-theanine attenuates the jitteriness of caffeine
• EGCG: Potential neuroprotective effects via anti-amyloid aggregation activity (preclinical)

7. Anti-Carcinogenic (Preclinical Mechanisms)

• Inhibition of cancer cell proliferation through cell cycle arrest (G0/G1 and G2/M phases)
• Induction of apoptosis in cancer cells via caspase activation
• Inhibition of angiogenesis (VEGF pathway)
• Epigenetic modulation: inhibition of DNA methyltransferases and histone deacetylases
• Inhibition of matrix metalloproteinases (anti-metastatic)
• Note: These mechanisms are primarily demonstrated in vitro and in animal models; clinical translation remains uncertain

Clinical Evidence Summary

Meta-Analyses — Lipid Effects

Xu et al. 2020 (Nutr J)

• Systematic review and meta-analysis of 31 RCTs, n=3,321 subjects
• Green tea supplementation significantly lowered:
  • Total cholesterol: WMD -4.66 mg/dL (95% CI: -6.36, -2.96; P < 0.0001)
  • LDL cholesterol: WMD -4.55 mg/dL (95% CI: -6.31, -2.80; P < 0.0001)
  • Triglycerides: WMD -3.77 mg/dL (not statistically significant overall; P = 0.15)
• No significant effect on HDL cholesterol
• Longer intervention duration associated with greater triglyceride reduction (WMD -9.03 mg/dL in longer-duration subgroup)
• Interpretation: Reductions correspond to approximately 2-5% improvements, which are modest but clinically meaningful in a population-level cardiovascular risk context
• [PubMed: 32434539]

Zheng et al. 2011 (Am J Clin Nutr)

• Meta-analysis of 14 RCTs in adults
• Green tea intake lowered fasting serum total cholesterol by 7.20 mg/dL (95% CI: -11.10, -3.27) and LDL cholesterol by 2.19 mg/dL (95% CI: -4.44, 0.05)
• [PubMed: 21715508]

Meta-Analyses — Blood Pressure

Peng et al. 2014 (Sci Rep)

• Meta-analysis of 13 RCTs, n=1,367 subjects
• Green tea consumption significantly decreased:
  • Systolic BP: -1.98 mmHg (95% CI: -2.94, -1.01; P < 0.001)
  • Diastolic BP: -1.92 mmHg (95% CI: -3.17, -0.68; P < 0.001)
• Effects were more pronounced in studies with longer durations (>12 weeks)
• [Source: Nature Scientific Reports, doi:10.1038/srep06251]

Nogueira et al. 2022 (Front Nutr)

• Systematic review and meta-analysis of 55 RCTs with 63 effect sizes
• Confirmed significant reductions in systolic BP (MD: -1.94 mmHg; 95% CI: -2.95 to -0.93; I2 = 8%; P = 0.0002)
• Also found improvements in fasting blood glucose and C-reactive protein
• [Source: Frontiers in Nutrition, doi:10.3389/fnut.2022.1084455]

Meta-Analyses — Body Weight and Composition

Hursel et al. 2009 (Int J Obes)

• Meta-analysis of RCTs examining catechins or EGCG-caffeine mixtures for weight loss
• Catechins significantly decreased body weight and maintained body weight after a period of weight loss
• Effect was small but statistically significant
• [PubMed: 19597519]

Asbaghi et al. 2024 (Br J Nutr)

• GRADE-assessed systematic review and dose-response meta-analysis of 59 RCTs, n=3,802
• Green tea extract supplementation significantly reduced:
  • Body mass and BMI
  • Body fat percentage
  • Malondialdehyde (oxidative stress marker)
• Significant increase in adiponectin and total antioxidant capacity
• Waist circumference: WMD -0.99 cm (from 13 studies)
• [PubMed: 38031409]

Cancer Prevention

Liu et al. 2025 (Am J Chin Med)

• Systematic review and meta-analysis: 43 studies (7 RCTs, 36 cohort studies) through October 2023
• Green tea consumption: RR 0.91 (95% CI: 0.86-0.96) for overall cancer risk
• EGCG supplementation: RR 0.72 (95% CI: 0.54-0.97)
• Notable site-specific reductions: prostate cancer (RR 0.43), oral cancer (RR 0.44), gallbladder cancer (RR 0.72), hematological cancers (RR 0.72)
• Dose-response analysis: significant negative linear correlation between higher green tea intake and cancer risk
• Limitation: Most evidence is observational; individual RCTs have shown mixed results
• [PubMed: 40832777]

Randomized Trial — Prostate Cancer (Bettuzzi et al. 2006)

• RCT of 600 mg/day green tea catechins vs. placebo for 1 year in men with high-grade prostatic intraepithelial neoplasia (HGPIN)
• n=60; only 1/30 (3%) in catechin group progressed to prostate cancer vs. 9/30 (30%) in placebo
• Small study but significant finding (P < 0.01)
• [PubMed: 16482057]

Randomized Trial — Prostate Cancer (Kumar et al. 2015)

• Larger RCT of 400 mg EGCG/day vs. placebo for 1 year in men with HGPIN or ASAP
• n=97; no significant difference in prostate cancer incidence between groups
• [PubMed: 25873370]

Glycemic Control

Liu et al. 2013 (Am J Clin Nutr)

• Meta-analysis of 17 RCTs examining green tea effects on glucose control and insulin sensitivity
• Green tea significantly reduced fasting glucose by -1.48 mg/dL (95% CI: -2.57, -0.40)
• No significant effects on fasting insulin, HbA1c, or HOMA-IR in the overall pooled analysis
• [PubMed: 23803878]

Randomized Trial — Type 2 Diabetes (Hsu et al. 2011)

• Double-blind, placebo-controlled RCT; 1,500 mg decaffeinated green tea extract/day for 16 weeks
• n=68 subjects with type 2 diabetes and BMI > 25
• No statistically significant differences detected between green tea extract and placebo groups in any measured variable
• [PubMed: 21649457]

Cognitive Function

Park et al. 2011 (J Med Food)

• Double-blind, placebo-controlled RCT of green tea extract + L-theanine combination (LGNC-07) in mild cognitive impairment
• Improvements in memory and attention
• [PubMed: 21303262]

Owen et al. 2008 (Nutr Neurosci)

• The combination of L-theanine (97 mg) and caffeine (40 mg) significantly improved accuracy during task switching and self-reported alertness, while reducing tiredness
• [PubMed: 21040626]

Baba et al. 2021 (J Med Food)

• RCT of L-theanine in middle-aged and older subjects
• Single dose reduced reaction time on attention tasks (Stroop test) and improved working memory accuracy
• [PubMed: 33751906]

Safety Profile

Contraindications

• Liver disease or hepatic impairment: High-dose green tea extracts (particularly those concentrated in EGCG) are contraindicated in patients with pre-existing liver conditions due to hepatotoxicity risk
• Iron deficiency anemia: Green tea tannins and polyphenols significantly reduce nonheme iron absorption; avoid concurrent consumption with iron-rich meals or iron supplements
• Hypersensitivity to Camellia sinensis or any component of the preparation
• Caffeine sensitivity: Individuals with anxiety disorders, cardiac arrhythmias, or insomnia should use caffeine-free or low-caffeine preparations

Drug Interactions

Nadolol (Beta-Blocker) — Clinically Significant

• Green tea markedly decreased the Cmax and AUC of nadolol by approximately 85% in healthy subjects through inhibition of intestinal OATP1A2 uptake transporter and enhancement of P-glycoprotein efflux
• Clinical consequence: significantly reduced blood pressure-lowering effect of nadolol
• Recommendation: Patients taking nadolol should avoid green tea or separate intake by several hours
• [PubMed: 24419562]

Anticoagulants (Warfarin)

• Green tea leaves contain vitamin K (approximately 1,650 mcg per 100 g of dried leaves), which can antagonize warfarin’s anticoagulant effect
• A published case report documented reduced INR with excessive green tea consumption (approximately 1 gallon/day)
• Moderate consumption (1-2 cups/day) is unlikely to affect INR significantly, but patients on warfarin should maintain consistent green tea intake
• [PubMed: 10332534]

Iron Supplements

• Tannins and catechins chelate nonheme iron, reducing absorption by up to 25-60%
• Separate green tea intake from iron supplements by at least 1-2 hours

General CYP450 and Transporter Interactions

• A 2025 review in Clinical Pharmacology & Therapeutics found that 72% of pharmacokinetic analyses reported significant decreases (18-99%) in systemic drug exposure with green tea co-consumption
• Green tea catechins inhibit OATP1A2, OATP2B1 uptake transporters and modulate P-glycoprotein efflux
• Weak inhibition of CYP3A4 and CYP1A2 at high doses
• Potential interactions with substrates of these transporters/enzymes: fexofenadine, statins (particularly rosuvastatin and simvastatin), and various other drugs
• [PubMed: 39748104]

Other Interactions

• Stimulant drugs/caffeine-containing products: Additive CNS stimulation
• MAO inhibitors: Theoretical potentiation of tyramine-related hypertensive crisis (due to caffeine content)
• Antiplatelet agents: Mild additive effect (weak antiplatelet activity of catechins)

Side Effects

• Common: GI discomfort (nausea, stomach upset, constipation or diarrhea), insomnia, restlessness, increased urination — most side effects are attributable to caffeine content
• Uncommon: Headache, dizziness, heartburn
• Rare but serious: Hepatotoxicity (primarily with high-dose concentrated extracts; see below)

Hepatotoxicity Risk — Important Safety Concern

The hepatotoxicity of green tea extracts has been the subject of multiple regulatory reviews:

EFSA Scientific Opinion (2018):

• Concluded that EGCG doses equal to or above 800 mg/day from food supplements have been shown to induce statistically significant increases in serum transaminases compared to control
• Below 800 mg/day, no evidence of hepatotoxicity was found in clinical trials for up to 12 months of use
• However, EFSA noted it was not possible to identify a universally safe dose, as one product containing 375 mg EGCG was associated with liver injury in a single case
• Green tea infusions consumed in the traditional manner are generally considered safe
• Fasting state increases risk: Consumption of concentrated extracts on an empty stomach is more likely to lead to adverse effects
• [PubMed: 32625874]

USP Comprehensive Review (2020):

• Analyzed 216 case reports on green tea products, including 34 concerning liver damage
• 27 reports categorized as possible causality, 7 as probable causality
• The Drug-Induced Liver Injury Network identified 40 cases directly attributable to green tea extract consumption out of 1,414 patients with confirmed liver injury (2004-2018)
• GTE-induced liver injury is likely idiosyncratic rather than dose-dependent in most cases
• COMT and UGT1A4 genotype polymorphisms may increase susceptibility
• [PubMed: 32140423]

Clinical Guidance:

• Use beverage-form green tea (traditional infusion) when possible; this form has a long safety history
• If using concentrated extracts, do NOT exceed 800 mg EGCG/day
• Always take green tea extracts with food, never on an empty stomach
• Monitor liver function tests if using high-dose extracts for extended periods
• Discontinue immediately if signs of hepatotoxicity occur (jaundice, dark urine, abdominal pain, fatigue)

Pregnancy and Lactation

• Pregnancy (Category C): Green tea contains caffeine; excessive caffeine intake (>200-300 mg/day) during pregnancy is associated with increased risk of miscarriage and low birth weight. Green tea also reduces folate bioavailability through inhibition of dihydrofolate reductase. High-dose supplements should be avoided. Moderate consumption of 1-2 cups/day is likely acceptable but should be counted toward total daily caffeine allowance.
• Lactation: Caffeine passes into breast milk. Moderate green tea consumption (1-2 cups/day, approximately 50-60 mg caffeine) is generally considered compatible with breastfeeding. Higher amounts may cause infant irritability and sleep disturbance. Newborns metabolize caffeine more slowly than adults. Tannins may also impair maternal iron absorption during a period of increased iron demand. [Source: LactMed, NCBI NBK501847]

Clinical Dosage

Key considerations:

• The clinical evidence base encompasses a wide range of preparations and doses, making direct comparisons difficult
• For cardiovascular and metabolic benefits, most positive meta-analyses used 200-500 mg catechins/day for 8+ weeks
• The EFSA safety threshold of 800 mg EGCG/day from supplements should be strictly observed
• Green tea infusions prepared in the traditional manner have a centuries-long safety record and are not subject to the same hepatotoxicity concerns as concentrated extracts
• Caffeine content varies substantially between preparations and should be considered in the total daily caffeine budget

Sources

Regulatory Documents

• German Commission E Monograph: Camellia sinensis (in: Blumenthal M et al. The Complete German Commission E Monographs. Austin, TX: American Botanical Council; 1998)
• EMA/HMPC Community Herbal Monograph: Camelliae sinensis non fermentatum folium. EMA/HMPC/283630/2012 (November 2013)
• EMA/HMPC Assessment Report: Camellia sinensis (L.) Kuntze, non fermentatum folium. EMA/HMPC/283629/2012 (November 2013)
• EFSA Panel on Food Additives and Nutrient Sources. Scientific opinion on the safety of green tea catechins. EFSA Journal. 2018;16(4):5239. [PubMed: 32625874]

Meta-Analyses and Systematic Reviews

• Xu R et al. Effect of green tea consumption on blood lipids: a systematic review and meta-analysis of randomized controlled trials. Nutr J. 2020;19(1):48. [PubMed: 32434539]
• Zheng XX et al. Green tea intake lowers fasting serum total and LDL cholesterol in adults: a meta-analysis of 14 randomized controlled trials. Am J Clin Nutr. 2011;94(2):601-610. [PubMed: 21715508]
• Peng X et al. Effect of green tea consumption on blood pressure: a meta-analysis of 13 randomized controlled trials. Sci Rep. 2014;4:6251. [doi: 10.1038/srep06251]
• Nogueira LP et al. The effects of green tea supplementation on cardiovascular risk factors: a systematic review and meta-analysis. Front Nutr. 2022;9:1084455.
• Hursel R et al. The effects of green tea on weight loss and weight maintenance: a meta-analysis. Int J Obes (Lond). 2009;33(9):956-961. [PubMed: 19597519]
• Asbaghi O et al. The effects of green tea extract supplementation on body composition, obesity-related hormones and oxidative stress markers: a GRADE-assessed systematic review and dose-response meta-analysis. Br J Nutr. 2024;131(7):1125-1157. [PubMed: 38031409]
• 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;98(2):340-348. [PubMed: 23803878]
• Liu et al. Green tea and epigallocatechin gallate (EGCG) for cancer prevention: a systematic review and meta-analysis. Am J Chin Med. 2025. [PubMed: 40832777]

Key Clinical Trials

• Bettuzzi S et al. Chemoprevention of human prostate cancer by oral administration of green tea catechins in volunteers with high-grade prostate intraepithelial neoplasia. Cancer Res. 2006;66(2):1234-1240. [PubMed: 16482057]
• Kumar NB et al. Randomized, placebo-controlled trial of green tea catechins for prostate cancer prevention. Cancer Prev Res (Phila). 2015;8(10):879-887. [PubMed: 25873370]
• Hsu CH et al. Does supplementation with green tea extract improve insulin resistance in obese type 2 diabetics? A randomized, double-blind, and placebo-controlled clinical trial. Altern Med Rev. 2011;16(2):157-163. [PubMed: 21649457]
• Park SK et al. A combination of green tea extract and L-theanine improves memory and attention in subjects with mild cognitive impairment. J Med Food. 2011;14(4):334-343. [PubMed: 21303262]
• Baba Y et al. Effects of L-theanine on cognitive function in middle-aged and older subjects: a randomized placebo-controlled study. J Med Food. 2021;24(4):333-341. [PubMed: 33751906]

Safety References

• Hu J et al. United States Pharmacopeia (USP) comprehensive review of the hepatotoxicity of green tea extracts. Toxicol Mech Methods. 2020;30(1):1-19. [PubMed: 32140423]
• Mazzanti G et al. Hepatotoxicity of green tea: an update. Arch Toxicol. 2015;89(8):1175-1191. [PubMed: 25975988]
• Isomura T et al. Liver-related safety assessment of green tea extracts in humans: a systematic review of randomized controlled trials. Eur J Clin Nutr. 2016;70(11):1221-1229. [PubMed: 27188915]
• Misaka S et al. Green tea ingestion greatly reduces plasma concentrations of nadolol in healthy subjects. Clin Pharmacol Ther. 2014;95(4):432-438. [PubMed: 24419562]
• Taylor JR, Wilt VM. Probable antagonism of warfarin by green tea. Ann Pharmacother. 1999;33(4):426-428. [PubMed: 10332534]

Mechanism of Action

• Lorenz M et al. A constituent of green tea, epigallocatechin-3-gallate, activates endothelial nitric oxide synthase by a PI3K-, PKA-, and Akt-dependent pathway. J Biol Chem. 2004;279(7):6190-6195. [PubMed: 14645258]
• Singh BN et al. Green tea catechin, epigallocatechin-3-gallate (EGCG): mechanisms, perspectives and clinical applications. Biochem Pharmacol. 2011;82(12):1807-1821. [PMC: 4082721]

Connections

• Compare with Garlic for another herb with modest but consistent cardiovascular risk factor reduction and drug interaction considerations
• See Olive Leaf for another plant-derived polyphenol with antihypertensive and lipid-modulating properties
• Compare hepatotoxicity concerns with Turmeric/Curcumin, another widely used polyphenol supplement with emerging liver safety questions at high doses
• See Ginkgo for another herb with antioxidant and vascular effects that has undergone large-scale clinical trials
• The L-theanine cognitive effects connect to the broader category of nootropic herbs; see Bacopa for another herb studied for cognitive enhancement