Camellia sinensis (Tea Plant): A Comprehensive Reference
1. Identity and Botanical Description
Camellia sinensis (L.) Kuntze — also known historically as Thea sinensis L. — is the botanical source of all true teas. It is a plant species in the family Theaceae, commonly known as the tea plant, and is the source of various types of tea including green tea, black tea, and oolong tea. Two primary botanical varieties account for nearly all commercial cultivation: C. sinensis var. sinensis (China tea) and C. sinensis var. assamica (Assam tea). The former is grown in China, Japan, and Taiwan, while the latter predominates in South and Southeast Asia, including Australia and Africa.
Morphologically, tea var. sinensis is an evergreen, multi-stemmed shrub that grows up to 3 m in height, while tea var. assamica can grow up to 10–15 m tall with one main stem. Under cultivation, young leaves of C. sinensis are regularly picked and tea plants are pruned and trained to a low, profusely branching and spreading bush of 1.0–1.5 m in height. Its leaves are alternate, exstipulate, lanceolate to obovate, up to 30 cm long, 2–5 cm broad, pubescent, sometimes becoming glabrous, serrate, acute or acuminate. Fresh leaves are often picked from late March and early April to July every year.
The actual tea plant is an evergreen tree native to the part of Southeast Asia where China's Yunnan Province meets India's Nagaland region and the northern areas of Myanmar, Thailand, Laos, and Vietnam. Long cultivated in China, it was also cultivated in Indonesia since the 18th century, and in India and Sri Lanka since the 19th century.
2. Common Forms and Preparations
All six classic types of tea — white tea, green tea, yellow tea, oolong, black tea, and pu-erh — are produced from Camellia sinensis. What distinguishes the different teas is processing, not the plant itself. When Camellia sinensis leaves oxidize, they become black tea; oolong leaves are partially oxidized; and green tea leaves remain unoxidized.
Green tea is prepared by steaming and pan-frying the Camellia sinensis leaves and then drying them. Because green tea is not fermented, it maintains important molecules called polyphenols, which are responsible for many of its benefits. All types of tea — green, black, oolong, and white — are produced from the Camellia sinensis plant using different methods. Tea is usually brewed and drunk as a beverage, but green tea extracts are also sold in capsules and sometimes used in skin products.
As a dietary supplement, Camellia sinensis is available in multiple forms:
- Brewed leaf tea: A standard cup of brewed green tea (1.5–2 grams of tea steeped for 3–5 minutes in recently boiled water) provides approximately 30–60 mg of EGCG and 20–45 mg of caffeine.
- Standardized dry extracts (capsules/tablets): concentrated preparations often standardized to a defined percentage of total catechins or EGCG, used in clinical studies and sold as dietary supplements.
- Topical ointment (prescription drug): Sinecatechins ointment, 15%, is manufactured from the extract of green tea leaves from Camellia sinensis, consisting of 85–95% catechins, with EGCG representing the primary catechin at more than 55%.
- Matcha: finely ground whole-leaf green tea powder consumed as a suspension.
Green tea represents about 20% of the dried tea manufactured annually and is mainly consumed in Asian countries, such as Japan, owing to its relative safety and cheaper price in comparison to other beverages.
3. Traditional and Historical Use
The tea plant was first cultivated in China thousands of years ago, and there is evidence of the use of tea for health purposes dating back about 3,000 years. A legendary account places the discovery of tea with the mythological Emperor Shen Nong: while water was being boiled in the emperor's garden, Camellia sinensis leaves blew into the pot, and Shen Nong drank the resulting beverage. With his first sip, he exclaimed "t'sa," meaning "godlike," and thus the name "cha" was adopted for the drink. Tea cultivation, however, did not begin in China until around 350 CE, and in 780 CE, Lu Yu published one of the first books about tea, which included descriptions of tea leaf shapes and tea ceremonies.
Anthropological research indicates that Camellia sinensis leaves were originally wild-harvested and consumed as a bitter vegetable that was cooked into nourishing soups and as a folk medicine prepared as a vitality tonic.
Tea was recognized by Chinese people as an effective traditional drink required for the prophylaxis against many health ailments. In Traditional Chinese Medicine (TCM), tea — called cha — has been used broadly across Chinese and most Asian cultures. The name changed to "chai" as tea spread into the Middle East. In the early twelfth century, merchants brought tea to the Muslim world, where it was consumed in place of wine and other forbidden stimulants and beverages.
Tea culture spread to Japan and developed into distinct traditions there: tea culture in Japan from as far back as the 9th century has resulted in various types. Chinese green teas are typically roasted or pan-fired to prevent oxidation, resulting in a rich, smoky or toasty flavor and earthy aromas. Japanese green teas are steamed, not roasted like Chinese green teas. Legally considered a beverage and not a pharmaceutical product, tea serves as a stimulant because of its caffeine content and can be used as an antidiarrheal because of its tannin content.
4. Key Constituents and Active Compounds
The complex chemical composition of green tea comprises different classes of chemical compounds, such as polyphenols, alkaloids, proteins, minerals, vitamins, amino acids, and others. Fresh leaves of Camellia sinensis have been reported to contain 22.2% polyphenols, 17.2% protein, 4.3% caffeine, 27.0% crude fiber, 0.5% starch, 3.5% reducing sugars, 6.5% pectins, 2.0% ether extract, and 5.6% ash.
4.1 Polyphenols / Catechins (Flavan-3-ols)
One third part of bioactive compounds in green tea is contributed by polyphenols, the most interesting constituent. The main type of polyphenols are called catechins (flavan-3-ols), also known as tannins, which serve as astringency constituents. The principal catechins in its leaves are epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), gallocatechin (GC), epicatechin (EC), and catechin.
Epigallocatechin-3-gallate (EGCG) is the most abundant polyphenol in green tea. Most important kinds of catechins include epigallocatechin-3-gallate (EGCG), comprising approximately 59% of total catechins, and epigallocatechin (EGC) at approximately 19%. The most extensively studied species, Camellia sinensis, has been shown to contain abundant phenolic compounds, including catechins, flavonol glycosides, and phenolic acid derivatives, which are largely responsible for its well-documented antioxidant and anti-inflammatory activities.
Approximately 10% of the green tea extract is composed of catechins; of these, epigallocatechin-3-gallate (EGCG) is present in the highest concentration. Processing is critical to catechin content: GTE catechin profiles vary significantly with manufacturing processes.
4.2 Methylxanthines (Caffeine and Related Alkaloids)
The leaves' main methylxanthine constituents include up to 4% caffeine (also called theine). The PubMed-indexed isolation study of C. sinensis var. assamica identified caffeine and theobromine among its characterized compounds. The amino acid L-theanine, found almost exclusively in tea, is another significant neuroactive constituent. Differences in the molar ratio of caffeine + EGCG to theanine + arginine in green tea have been shown to affect stress reduction and sleep in experimental and clinical studies. EGCG and theanine are unique ingredients in green tea that affect brain function.
4.3 Other Phytochemical Constituents
Chemical compounds found in tea flowers include flavonols, catechins, polysaccharides, saponins, proteins, alkaloids, spermidine derivatives, and anthocyanins. These include catechins, flavonol glycosides, and phenolic acid derivatives. Additionally, from the seeds, researchers have isolated gallic acid, (+)-catechin, ampelopsin, (-)-epicatechin, and epicatechin-3-O-gallate, among other compounds.
5. Mechanisms of Action
5.1 Antioxidant Activity
EGCG is the major and the most active component in green tea. Studies have shown that EGCG protects cellular damage by inhibiting DNA damage and oxidation of LDL. One of the protective properties of EGCG is its ability to scavenge free radicals. The protective effect of EGCG is due to its ability to decrease lipid peroxidation, oxidative stress, and the production of nitric oxide (NO) radicals by inhibiting the expression of iNOS.
5.2 Anti-inflammatory Action
Green tea/EGCG acts as an antioxidant to scavenge reactive oxygen species, leading to attenuation of nuclear factor-κB (NF-κB) activity. EGCG also ameliorates the overproduction of pro-inflammatory cytokines and mediators, and reduces the activity of NF-κB and AP-1. Green tea and EGCG suppress the gene and/or protein expression of inflammatory cytokines and inflammation-related enzymes.
5.3 Modulation of Cell Signaling and Apoptosis
Thanks to multiple interactions with cell surface receptors, intracellular signaling pathways, and nuclear transcription factors, EGCG possesses a wide variety of anti-inflammatory, antioxidant, antifibrotic, anti-remodeling, and tissue-protective properties. EGCG has been described as a potent inducer of apoptosis and an inhibitor of telomerase activity. Anti-apoptotic protein B-cell lymphoma 2 (Bcl-2) is suppressed by EGCG through down-regulation of NF-κB, which can explain the apoptosis-inducing property of EGCG in its anti-cancer effect.
5.4 Effects on Drug-Metabolizing Enzymes and Transporters
Catechins are the abundant polyphenolic compounds in green tea (e.g., EGCG), which are reported to influence determinants of drug pharmacokinetics, such as drug solubility and the activity of drug transporters and drug-metabolizing enzymes. For most drugs reporting an interaction, green tea catechins were proposed to decrease intestinal drug absorption by inhibiting OATP uptake (particularly OATP1A2), enhancing P-glycoprotein (P-gp) efflux activity, or reducing drug solubility.
5.5 Metabolic / AMPK Pathway
Several lines of evidence have shown that EGCG can stimulate reactive oxygen species generation leading to the activation of AMP-activated protein kinase (AMPK) by phosphorylation. Phosphorylated AMPK generated by EGCG's action can modulate some proteins involved in adipogenesis, lipogenesis, and lipolysis.
6. Scientific Evidence by Area of Use
6.1 Cardiovascular Risk Factors
A bulk of observational studies have revealed the protective role of green tea supplementation in cardiovascular diseases. A systematic review and meta-analysis examined the effects of green tea supplementation on cardiovascular risk factors including lipid profile, blood pressure, glycemic control markers, and C-reactive protein (CRP). Among 11,286 studies initially identified, 55 eligible randomized controlled trials (RCTs) with 63 effect sizes were included. Results from the random effects meta-analysis showed that green tea extract (GTE) supplementation significantly reduced total cholesterol (WMD = −7.62 mg/dL; 95% CI: −10.51, −4.73), LDL-C (WMD = −5.80; 95% CI: −8.30, −3.30), fasting blood sugar (WMD = −1.67; 95% CI: −2.58, −0.75), HbA1c (WMD = −0.15; 95% CI: −0.26, −0.04), and diastolic blood pressure (WMD = −0.87; 95% CI: −1.45, −0.29), while increasing HDL-C (WMD = 1.85; 95% CI: 0.87, 2.84).
Evidence characterization: This meta-analysis covered a substantial number of RCTs, but subgroup analyses based on duration of supplementation (≥12 vs. <12 weeks), dose of GTE (≥1,000 vs. <1,000 mg/d), sex, and baseline serum levels demonstrated different results for some risk factors, indicating that effect sizes are context-dependent. Effect magnitudes, while statistically significant, are modest. Very few long-term studies have looked at the effects of tea on heart disease risk.
6.2 Obesity and Body Weight
Green tea and its extracts are promoted as dietary supplements for losing weight, reducing blood cholesterol levels, and preventing chronic diseases such as heart disease and cancer. Research indicates that green tea extract (GTE) supplementation is investigated for a range of conditions, including several forms of cancer, CVD, and liver diseases; nevertheless, the existing evidence addressing its effects on body composition, oxidative stress, and obesity-related hormones is inconclusive.
Evidence characterization: Green tea has not been shown to be effective for weight loss according to NCCIH. The overall evidence on GTE and body weight is inconsistent across RCTs. The NCCIH further notes that although many studies have been done on green tea and its extracts, definite conclusions cannot yet be reached on whether green tea is helpful for most of the purposes for which it is used.
6.3 Cancer — Chemoprevention
Many studies suggest an inverse relationship between green tea intake and the risk of a variety of cancers, although other studies have found no association. Clinical trials have been small and heterogeneous with contradictory results. Mixed findings were observed for breast, esophageal, gastric, and liver cancers, with a mostly null association for colorectal, pancreatic, and prostate cancer.
In preclinical models, EGCG enhances gap junctional communication between adjacent cells and thus protects the cells from tumor development; the anti-tumor effects of EGCG are also partially related to its wide anti-inflammatory and antioxidant effects, as EGCG may suppress chronic inflammatory processes resulting in cell transformation and hyperproliferation.
Evidence characterization: The FDA's position is that evidence linking green tea consumption to cancer prevention is "weak and limited." Results from human studies are not always positive, possibly due to the fact that higher doses of tea are used in animal studies than those consumed by humans, and experimental conditions in animal studies are generally optimized. Overall, cancer prevention evidence remains preliminary and no regulatory body has approved green tea as a cancer-preventive agent based on clinical evidence.
6.4 Genital Warts (HPV) — FDA-Approved Indication
This is the one area where a Camellia sinensis-derived product holds regulatory approval. Polyphenon E was developed as a standardized green tea catechin preparation and was approved by the Food and Drug Administration of the United States in 2006 as a medication to treat genital warts (sold as Veregen® or sinecatechins). Sinecatechins ointment, 15%, is manufactured from the extract of green tea leaves from Camellia sinensis, consisting of 85–95% catechins. It is the first botanical agent approved by the FDA for prescription use and is indicated for the topical patient-applied treatment of external genital and perianal warts. Two Phase 3 clinical trials involving more than 1,000 male and female patients with external genital warts (EGW), treated for up to 16 weeks with sinecatechins ointment 15%, resulted in statistically superior complete clearance rates of all warts compared with vehicle-treated patients (54.9% vs. placebo).
The clearance rate of wart lesions after treatment with sinecatechins is similar to other topically applied drugs like imiquimod and podophyllotoxin, but recurrences are seen less frequently after treatment with sinecatechins. The molecular mode of action has not been fully characterized, but the following properties are likely involved in the regression of genital warts: activation of cellular immune reactions, induction of cell cycle arrest and apoptosis, and inhibition of HPV gene expression.
6.5 Cognitive Function and Neuroprotection
A number of experimental studies in vitro and in vivo have shown the neuroprotective effects of green tea and its components, such as catechins and theanine. In a human pilot clinical study, twelve elderly nursing home residents with cognitive dysfunction (mean age 88 years) consumed green tea powder 2 g/day for 3 months. After three months of green tea consumption, participants' MMSE-J scores were significantly improved (before: 15.3 ± 7.7; after: 17.0 ± 8.2; p = 0.03). However, this result suggests that green tea consumption may be effective in improving cognitive function or reducing the progression of cognitive dysfunction, but long-term large-scale controlled studies are needed to further clarify the effect.
A Japanese study found that daily consumption of 336.4 mg of decaffeinated green tea catechins may improve cognitive function in Japanese adults aged 50–69 with cognitive decline and a MMSE score of >24. Daily GTC consumption significantly improved cognitive function, suggesting the potential for improved working memory. Conversely, another study found that green tea or EGCG did not significantly impact cognitive functioning in participants given 2 g/day of green tea powder or a placebo, illustrating the mixed nature of the current evidence.
Evidence characterization: Human evidence is preliminary. While epidemiological data is suggestive, RCTs are small, short-term, and heterogeneous. Most mechanistic work is from animal and cell models. Large, well-controlled trials are lacking.
6.6 Type 2 Diabetes and Glycemic Control
Pharmacological evidence and insights exist into the antidiabetic mechanisms of green tea via experimental and clinical models, encouraging its use as a complementary nutraceutical in managing the biochemical alterations in the onset and progression of diabetes. The meta-analysis of RCTs described in Section 6.1 also found significant reductions in fasting blood sugar and HbA1c with GTE supplementation (WMD = −1.67 for FBS; WMD = −0.15 for HbA1c), though effect sizes were small.
High-fat-diet-induced increases in inflammatory TNF-α levels in rat islets were attenuated by supplementation of EGCG, suggesting that EGCG's anti-diabetic effect may be mediated through suppression of inflammation by modulation of NF-κB activity.
Evidence characterization: Mechanistic evidence is strong at the preclinical level. Human RCT evidence shows statistically significant but modest effects on glycemic markers. Evidence is not sufficient to support using green tea as a treatment for diabetes.
6.7 Skin and Photoprotection
A systematic review found few clinical studies, including five focused on UV-induced erythema and skin alterations, three on photoaging, two on antioxidant skin defenses, and one on acne and genodermatosis. Despite several proposed benefits, clinical evidence only supports the use of oral green tea preparations to protect skin from damage induced by ultraviolet radiation; in other cases, conflicting results and methodological limits do not allow clarification of efficacy.
6.8 Inflammatory Diseases
Most of the results from human studies indicated the beneficial effects of green tea and tea catechins against inflammatory diseases. Cellular and animal studies also provided evidence for the favorable effects of green tea/EGCG. Since green tea and EGCG have multiple targets and act in a pleiotropic manner, their usage may improve the quality of life in patients with inflammatory disease.
In the context of multiple sclerosis, an analysis of EGCG plus glatiramer acetate (GA) in 122 relapsing-remitting MS (RRMS) patients found that the 800 mg/day oral EGCG plus GA dose for an 18-month duration revealed no significant improvement in MRI and clinical activities compared to placebo.
7. Body Systems and Health Areas of Association
- Cardiovascular system: Lipid profile (total cholesterol, LDL-C, HDL-C), blood pressure, endothelial function
- Metabolic system: Glycemic control, insulin sensitivity, adipogenesis, body weight
- Central nervous system: Cognitive function, neuroprotection, neuroinflammation, potential role in neurodegenerative disease prevention
- Integumentary system: UV-induced skin damage, photoaging, HPV-related warts (topical FDA-approved use)
- Immunological system: Anti-inflammatory effects via NF-κB and cytokine pathways, immunomodulation
- Oncology: Chemopreventive investigation across multiple cancer sites (evidence remains preliminary in humans)
- Hepatic system: Paradoxically, both potential protective effects (beverage consumption) and hepatotoxic risk (high-dose extracts) have been documented
- Gastrointestinal system: Historical use as antidiarrheal; some evidence related to gut microbiome modulation
8. Dosage Forms and Dosages Reported in Studies
Human clinical studies demonstrate that single doses of up to 1.6 grams of green tea extract are well tolerated. The maximum tolerated dose in humans is reported to be 9.9 grams per day, a dose equivalent to 24 cups of green tea.
Specific dosages used in cited human studies include:
- Green tea powder: 2 g/day for 3 months (cognitive function pilot study, elderly residents).
- Decaffeinated green tea catechins: 336.4 mg/day (cognitive function study in adults aged 50–69).
- Decaffeinated green tea extracts: 800 mg/day of EGCG in a four-week clinical safety study in healthy individuals.
- Daily intake of 1,315 mg of green tea catechins containing 843 mg EGCG assessed in clinical research.
- 600 mg EGCG per day in RRMS patients for 12 months was investigated for antioxidant and metabolic effects.
- Topical Sinecatechins (Veregen®): 15% ointment applied to genital warts for up to 16 weeks (Phase 3 trials).
Subgroup analyses suggest that a dose of GTE ≥1,000 mg/day may produce different results than <1,000 mg/day for some cardiovascular risk factors.
The EFSA Safety Panel concluded that catechin doses of 800 mg per day or above from supplements are associated with a risk of liver damage — a threshold easily exceeded by some commercially available products.
9. Safety Considerations and Drug Interactions
9.1 Hepatotoxicity — The Primary Safety Concern
Drinking green tea has not been associated with liver injury or serum aminotransferase elevations; indeed, cross-sectional studies suggest that regular use of green tea is associated with lower serum ALT and AST values. However, concentrated extracts present a meaningfully different risk profile. Case series and a systematic review by the United States Pharmacopeia have raised the issue of the potential for green tea extract to cause hepatotoxicity.
A systematic review found 34 reports of liver damage, ranging from acute hepatitis to fulminant liver failure requiring transplant, following the use of multiple green tea extract preparations. As a result, the United States Pharmacopeia have suggested, but not mandated, a warning stating that symptoms of liver injury be placed on any green tea extract monograph produced.
The proposed mechanism is well characterized: the mechanism of hepatotoxicity from GTE has been attributed to the effects of EGCG, which results in reactive oxygen species formation due to decoupling of oxidative phosphorylation mechanisms. This effect is potentiated by chronic intake and fasting states, primarily due to prolonged duodenal transit time and impaired hepatic microsomal enzyme activity consequent to a calorie-deficit state.
A genetic susceptibility factor has also been identified: the close association of liver injury from green tea with the HLA allele B*35:01 suggests an immunologic etiology. Published adverse event case reports associate hepatotoxicity with EGCG intake amounts from 140 mg to approximately 1,000 mg/day, with substantial inter-individual variability in susceptibility, possibly due to genetic factors.
Based on these findings, the USP included a cautionary labeling requirement in its Powdered Decaffeinated Green Tea Extract monograph that reads: "Do not take on an empty stomach. Take with food." Additionally, the green tea extract Exolise was withdrawn from the market in France and Spain in 2003 following safety concerns.
Side effects of high doses of green tea extract are usually mild and include headache, dizziness, and nausea. The safety and tolerability of long-term use of green tea extracts has not been well defined.
The EFSA further concluded that the risk of liver injury appears to be higher when supplements are taken on an empty stomach or as a single high bolus dose. The European Medicines Agency (EMA) Committee on Herbal Medicinal Products has also issued a public statement highlighting the risk of liver injury associated with green tea extracts.
9.2 Drug Pharmacokinetic Interactions
A 2025 systematic review published in Clinical Pharmacology & Therapeutics found that the majority of analyses (72%) reported significant decreases (by 18–99%) in systemic drug exposure with green tea consumption across drugs including atorvastatin, celiprolol, digoxin, fexofenadine, folic acid, lisinopril, nadolol, nintedanib, raloxifene, and rosuvastatin. One analysis (6%) reported a 50% increase in drug systemic exposure (sildenafil), and for 22% of analyses drug pharmacokinetics were not affected by green tea consumption (fluvastatin, pseudoephedrine, simvastatin, and tamoxifen).
Notable specific interactions include:
- Warfarin/anticoagulants: Green tea supplements can interact with warfarin, where the effect on INR is complex and unpredictable. The vitamin K naturally present in tea leaves may partially counteract anticoagulant effects.
- Non-heme iron: The catechins in green tea inhibit the absorption of nonheme iron, making iron supplementation less effective for those with iron-deficiency anemia.
- CYP1A2 substrates: Green tea can block the CYP1A2 enzyme and may raise drug levels. Lab and clinical data suggest that strong CYP1A2 inhibitors can significantly increase levels of fezolinetant (Veozah). Because green tea and green tea extracts can inhibit CYP1A2, using them with Veozah is not recommended.
- Milk proteins and bioavailability: Tea catechins can bind to milk proteins and form a network of casein micelles. For EGCG, the galloyl functional group is responsible for this affinity through hydrogen bonds and hydrophobic interactions. The free catechins that could be absorbed in vivo from green tea infusions could be influenced by these interactions with milk proteins.
9.3 Variability in Product Composition
There is great variability in the concentration of green tea extract, EGCG, and other components among marketed products, which may explain why some products have been implicated in hepatotoxicity.
9.4 Caffeine-Related Effects
The caffeine in green tea combined with other stimulants like ADHD medications can cause an additive effect, leading to elevated heart rate and blood pressure. Green tea is also a source of caffeine relevant to populations sensitive to this methylxanthine, including those with cardiovascular conditions, anxiety disorders, or those taking caffeine-metabolizing medications.
References
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