Green Tea

Camellia sinensis (L.) Kuntze belongs to Theaceae, the tea family, in the order Ericales; NCBI Taxonomy files it as taxon 4442 and lists the older name Thea sinensis as a synonym. Britannica describes it as an evergreen flowering plant valued for its young leaves and leaf buds. Two varieties dominate commercial growing: the small-leaved China plant (var. sinensis) and the large-leaved Assam plant (var. assamica), along with hybrids of the two; a third, the Cambodia variety, is not widely cultivated. The plant’s natural home is a fan-shaped area running from Nagaland, Manipur, and the Assam–Myanmar frontier east through China, probably as far as Zhejiang province, and south through the hills of Myanmar and Thailand into Vietnam. Most of the world’s tea is still grown within that native range, with significant production in Kenya and South America (Britannica).

Left alone, the varieties look very different. Britannica describes the China variety as a hardy, multi-stemmed bush up to about 2.75 m tall that withstands cold winters and stays productive for at least 100 years, and the Assam variety as a single-stemmed tree of 6–18 m with an economic life of about 40 years. In cultivation the plant is kept as a low, mounded shrub and pruned often to encourage fresh shoots. Mature leaves range from 3.8 to 25 cm long, smallest in the China variety, and may be serrated, blistered, or smooth, with varying amounts of fine hair. The fragrant white flowers have yellow centers and are about 4 cm across. Pickers usually take the bud and the two youngest leaves. Tea needs at least 1,140–1,270 mm of rain a year and acidic soil, around pH 5.8 to 5.4 or lower; it cannot be grown on alkaline ground.

Processing decides what kind of tea a leaf becomes. Britannica explains that in black tea manufacture the leaf is withered, rolled, ‘fermented’, and dried; the fermentation is really enzymatic oxidation, in which polyphenol oxidase turns colorless, bitter polyphenols reddish and creates the flavor of black tea. Green tea skips this step, and EFSA notes that this prevents oxidation of the polyphenols, most of which in green tea are catechins. Green tea is usually made from the China variety and grown mainly in Japan and China, while black tea is best made from Assam or hybrid plants. The main catechins are EGCG (PubChem CID 65064, C22H18O11), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC). Caffeine makes up about 4% of the solids in fresh leaf, and L-theanine (CID 439378, C7H14N2O3) is an amino acid found in tea.

Britannica records the legend that tea has been known in China since about 2700 BCE. For millennia it was a medicinal drink made by boiling fresh leaves, and around the third century CE it became an everyday beverage, with cultivation and processing beginning; the first published account of how to plant, process, and drink tea appeared in 350 CE. The great classic came later. Lu Yu (733–804), whom Ulrich Theobald’s ChinaKnowledge reference says was later called the God of Tea, wrote the Chajing, or Classic of Tea, around 760 under the Tang dynasty. In ten short chapters it describes the origin of the tea plant, the tools and methods for picking and processing leaves, the utensils for brewing, the choice of water, drinking customs, and the tea-growing regions of China and the quality of their leaf.

Britannica dates the arrival of the first tea seeds in Japan to around 800, with cultivation established by the thirteenth century. The decisive figure was the Zen monk Eisai, also called Yōsai (1141–1215). The Omotesenke tea school explains that at the end of the twelfth century he brought back from China both Zen Buddhism and a new way of making and drinking tea, much like the modern custom of whisking powdered green tea. The Ueda Sōko tea school dates his Kissa Yōjōki (Drinking Tea for Health), the first Japanese treatise on tea, to 1211; it opens by calling tea an elixir for health and long life. According to the Azuma kagami chronicle, as Omotesenke recounts, in 1214 Eisai prepared tea for the shogun Minamoto no Sanetomo, who was suffering from a hangover, and presented him with the book. Tea drinking then spread widely through Japan.

Europe came to tea through trade. Britannica records that the Dutch East India Company carried the first consignment of China tea to Europe in 1610, and the English East India Company brought China tea to London in 1669. In 1824 tea plants were found in the hills between Burma and Assam, and the British introduced tea growing into India in 1836 and Ceylon (Sri Lanka) in 1867, at first with seeds from China and later from the Assam plant. Today black tea is by far the most common type produced. Green tea’s catechins later became the basis of a medicine: in 2006 the FDA approved sinecatechins ointment, which Stockfleth and Meyer (Expert Opinion on Biological Therapy 2014) describe as the first botanical drug licensed in the United States. Concentrated extracts also brought reports of liver injury. After a systematic review of them, USP added a cautionary label requirement to its monograph for powdered decaffeinated green tea extract, and in 2018 EFSA published a scientific opinion on the safety of green tea catechins.

Most proposed benefits of green tea are attributed to its catechins, and EFSA calls EGCG the most relevant of them. MSKCC summarizes the proposed mechanisms: green tea may improve blood lipids, EGCG triggers nitric oxide production in blood vessel walls, which relaxes the vessels, and EGCG inhibits enzymes involved in cell replication and DNA synthesis in cancer cell studies. The blood vessel and cancer findings come mainly from laboratory studies and may not reflect what happens in people drinking tea. How much EGCG enters the body depends heavily on how it is taken: the USP review by Oketch-Rabah and colleagues (Toxicology Reports 2020) found that bolus doses of extract taken on an empty stomach markedly increase EGCG absorption, possibly by overwhelming the gut and liver’s first-pass clearance.

Caffeine is green tea’s best-understood active ingredient. It is a stimulant, and the FDA lists the effects of too much: a faster heart rate, palpitations, raised blood pressure, insomnia, anxiety, jitters, upset stomach, nausea, and headache. The Cochrane weight-loss review notes that both catechins and caffeine are believed to increase energy metabolism, which is the rationale for weight-loss products. L-theanine is the reason some people find tea calmer than coffee. Payne and colleagues (Nutrition Reviews 2025) pooled trials in healthy people and found small to moderate improvements in some measures of attention and mood in the first two hours after theanine combined with caffeine, compared with placebo. Their confidence intervals were often wide, and the authors called for more research using tea itself or tea-equivalent doses.

Severe liver injury from green tea appears to be idiosyncratic and immune-mediated, not simply a matter of dose. Hoofnagle and colleagues (Hepatology 2021), studying the US Drug-Induced Liver Injury Network, found that 72% of people with green tea liver injury carried the immune gene variant HLA-B*35:01, compared with about 11% of population controls; NCCIH estimates that 5–15% of Americans carry the variant. Several drug interactions involve transporters, the proteins that carry drugs into cells. Misaka and colleagues (Clinical Pharmacology and Therapeutics 2014) showed in cell studies that green tea blocks OATP1A2, a transporter that carries nadolol, which may partly explain lower nadolol absorption from the gut, and Clarke and colleagues (Clinical and Translational Science 2023) found that green tea extract inhibited 10 of 16 transporters tested. In laboratory and animal studies by Golden and colleagues (Blood 2009), EGCG reacted directly with the cancer drug bortezomib and blocked its effect.

Weight loss is the commonest reason people buy green tea extract, and the evidence is disappointing. Jurgens and colleagues (Cochrane 2012) reviewed randomized trials lasting at least 12 weeks in overweight or obese adults. In six trials outside Japan, with 532 participants, green tea preparations made almost no difference to weight: an average of 0.04 kg, with no significant change in body mass index or waist size. Eight Japanese trials with 1,030 participants were too different from each other to pool, showing losses of 0.2 to 3.5 kg. The reviewers concluded that green tea produces a small, statistically non-significant weight loss that is not likely to be clinically important, and that it does not help to maintain weight loss. NCCIH adds that any effect may depend on the product and on physical activity.

Cholesterol and blood pressure show small but fairly consistent effects. Xu and colleagues (Nutrition Journal 2020) pooled 31 randomized trials with 3,321 people and found that green tea lowered total cholesterol by about 4.7 mg/dL and LDL cholesterol by about 4.6 mg/dL, with no change in HDL cholesterol or triglycerides. NCCIH agrees that the effect is small and notes that most trials used extract supplements rather than tea. In 24 trials with 1,697 people, the same group (Medicine 2020) found systolic and diastolic blood pressure about 1.2 mmHg lower, over short-term trials, and called for longer studies. For comparison, the LDL drop is roughly a third of the 0.33 mmol/L (about 13 mg/dL) that Jovanovski and colleagues (American Journal of Clinical Nutrition 2018) pooled for about 10 g of psyllium a day. No trial has shown that green tea prevents heart attacks or strokes.

Population studies are more striking, but cannot prove cause and effect. In the Ohsaki study, Kuriyama and colleagues (JAMA 2006) followed 40,530 Japanese adults aged 40 to 79 for up to 11 years. Compared with less than a cup a day, five or more cups was linked to 12% lower all-cause mortality in men and 23% lower in women, and to fewer deaths from cardiovascular disease, but not from cancer. People who drink a lot of green tea may differ from others in many ways. NCCIH reports that green tea drinking has been linked to a lower risk of coronary heart disease in Asian but not Western populations. For blood sugar, Liu and colleagues (American Journal of Clinical Nutrition 2013) found small falls in fasting glucose and HbA1c across 17 trials, but a larger 2020 analysis of 27 trials by Xu and colleagues (Nutrition and Metabolism) found only a 1.4 mg/dL fall in fasting glucose and no significant change in HbA1c.

Cancer prevention has been studied for decades without a clear answer. Filippini and colleagues (Cochrane 2020) included 142 studies: 11 randomized trials with 1,795 participants and 131 observational studies with more than 1.1 million. Trials of green tea extract for prostate cancer gave a risk ratio of 0.50, with a confidence interval running from 0.18 to 1.36, which is too uncertain to show benefit. Across eight studies with 504,366 people, green tea drinking was not associated with cancer deaths. Results conflicted between study designs, most studies came from Asian populations, and the reviewers found limited evidence of benefit. MSKCC adds that a supplement combining high-dose green tea catechins with selenium and lycopene was associated with more prostate cancer in high-risk men, and that long-term EGCG did not reduce prostate cancer risk.

Two other uses deserve mention. For alertness, the Payne meta-analysis suggests that theanine with caffeine may briefly sharpen attention, but trials using actual cups of tea are few. For genital warts, the evidence is strong because sinecatechins is a licensed drug. Tatti and colleagues (Obstetrics and Gynecology 2008) randomized 502 adults to sinecatechins ointment 15% or 10%, or an inactive ointment, three times a day for up to 16 weeks. All warts cleared in 57.2% and 56.3% of the sinecatechins groups against 33.7% on the inactive ointment, and warts recurred during follow-up in 6.5% and 8.3%, against 8.8% on the inactive ointment. The FDA label limits Veregen to adults with healthy immune systems, and CDC treatment guidelines list it as a patient-applied option. Drinking tea or applying homemade preparations is not the same treatment.

As a drink, green tea is a mild source of caffeine. The FDA lists a typical 12-ounce serving of green tea at about 37 mg of caffeine, against 71 mg for black tea and 113–247 mg for brewed coffee, and cites 400 mg a day as an amount not generally associated with harm in most adults. EFSA estimates that European adults who drink green tea take in 90–300 mg of EGCG a day on average, and up to 866 mg for very heavy drinkers. Matcha is different from brewed leaf, because you drink the whole leaf: the Ueda Sōko tea school describes leaves shaded before harvest, steamed, dried, stripped of stems and veins, ground in a stone mill, and whisked with hot water, about 2–3 g of powder to 80–100 ml for thin tea.

Supplements are far more concentrated and much less predictable. EFSA found that green tea food supplements supply anywhere from 5 to 1,000 mg of EGCG a day, and the USP review notes that catechin profiles vary greatly with manufacturing. NCCIH warns that some weight-loss products combine green tea with other ingredients, and Hoofnagle and colleagues found that multi-ingredient products have become increasingly common causes of liver injury, with green tea a frequent component. MSKCC cites a one-year trial in which 200 mg of EGCG twice a day with food appeared safe, while 800 mg a day was associated with raised liver enzymes. USP now requires its decaffeinated green tea extract monograph to carry the warning: do not take on an empty stomach, take with food, and do not use if you have a liver problem.

Practical points: if you enjoy green tea, drinking it as tea is the form with the longest safety record. If you choose an extract, pick a single-ingredient product that states its EGCG content, keep well below 800 mg of EGCG a day, take it with food, and avoid multi-ingredient fat burners. Add up caffeine from all sources, including coffee, soft drinks, energy drinks, and caffeinated supplements. Stop at once and see a doctor if you develop abdominal pain, dark urine, or yellowing of the skin or eyes. Sinecatechins ointment is a prescription medicine applied to warts three times a day for no more than 16 weeks, and it is not interchangeable with green tea products sold as supplements.