Tea

The earliest clear reference to the medicinal use of tea dates to the Tang dynasty. Lu Yu’s The Classic of Tea set out the ceremonies of tea drinking and drew on Taoism and Confucianism ^{40Reference 40Barceloux · 2012Medical toxicology of drug abuse: synthesized chemicals and psychoactive plants}.

Green tea was considered to prolong life, and it was used for headache, body ache, indigestion, depression, low energy, obesity and hair loss ^{40Reference 40Barceloux · 2012Medical toxicology of drug abuse: synthesized chemicals and psychoactive plants}. Chinese folk remedies also used the tea flowers, including for inflammatory diseases ^{18Reference 18Chen et al. · 2012AnimalAnti-inflammatory effects of a polyphenols-rich extract from tea (Camellia sinensis) flowers in acute and chronic mice models — animalView study →}.

In Chinese medicine green tea is bitter, sweet and cold. It enters the Heart, Liver, Kidney, Stomach and Spleen channels, and its direction is descending. It cools heat, drains damp-heat and drains fire. It is contraindicated in cold patterns and Spleen deficiency ^{46Reference 46Hempen et al. · 2009A materia medica for Chinese medicine: plants, minerals, and animal products}.

Black tea is bitter and sweet in the same way, but warm rather than cold ^{46Reference 46Hempen et al. · 2009A materia medica for Chinese medicine: plants, minerals, and animal products}.

Tea is drunk ceremonially in several cultures, including the Chinese gongfu ceremony and the Japanese tea ceremony with powdered matcha. Gongfu brewing steeps the leaves for only a few seconds and reuses them many times.

Sri Lankan traditional practitioners have used black tea to promote urine flow ^{30Reference 30Abeywickrama et al. · 2010AnimalOral diuretic activity of hot water infusion of Sri Lankan black tea (Camellia sinensis L.) in rats — animalView study →}.

Tea reached Europe in the 17th century, at about the same time as coffee, and it became the more widely drunk of the two ^{40Reference 40Barceloux · 2012Medical toxicology of drug abuse: synthesized chemicals and psychoactive plants}.

Meta-analyses of randomised trials find small falls in blood pressure. Across 24 trials (1,697 people), green tea lowered systolic pressure by 1.17 mmHg and diastolic by 1.24 mmHg ^{1Reference 1Xu et al. · 2020Meta-analysisEffect of green tea supplementation on blood pressure: a systematic review and meta-analysis of 24 randomised controlled trialsView study →}. Two meta-analyses of 13 trials each found falls of about 2 mmHg systolic and 1.7–1.9 mmHg diastolic ^{2,3Reference 2Khalesi et al. · 2014Meta-analysisGreen tea catechins and blood pressure: a systematic review and meta-analysis of randomised controlled trialsView study →Reference 3Peng et al. · 2014Meta-analysisEffect of green tea consumption on blood pressure: a meta-analysis of 13 randomised controlled trialsView study →}.

A 2025 dose-response meta-analysis of 36 trials found falls of about 1.1 mmHg in both. It found no clear link between dose or duration and effect, and the authors regarded green tea as a complementary approach ^{4Reference 4Rezaei et al. · 2025Meta-analysisEffect of green tea supplementation on blood pressure in adults: a GRADE-assessed systematic review and dose-response meta-analysis of 36 randomised controlled trialsView study →}.

The effect was larger in people who started with raised blood pressure and in trials that used an extract ^{2Reference 2Khalesi et al. · 2014Meta-analysisGreen tea catechins and blood pressure: a systematic review and meta-analysis of randomised controlled trialsView study →}. In one analysis the benefit appeared only with low-dose polyphenols, longer trials, or when caffeine was accounted for ^{3Reference 3Peng et al. · 2014Meta-analysisEffect of green tea consumption on blood pressure: a meta-analysis of 13 randomised controlled trialsView study →}.

Gap: Effects are small and driven by concentrated extracts; whether 2-3 cups of everyday tea meaningfully changes clinic blood pressure is not established.

Green tea produces small, consistent falls in total and LDL cholesterol, with no change in HDL. In 14 trials (1,136 people), total cholesterol fell by 7.2 mg/dL and LDL by 2.2 mg/dL ^{5Reference 5Zheng et al. · 2011Meta-analysisGreen tea intake lowers fasting serum total and LDL cholesterol in adults: a meta-analysis of 14 randomised controlled trialsView study →}. A 31-trial analysis (3,321 people) found falls of 4.7 and 4.6 mg/dL ^{6Reference 6Xu et al. · 2020Meta-analysisEffect of green tea consumption on blood lipids: a systematic review and meta-analysis of 31 randomised controlled trialsView study →}.

In the first analysis the result did not depend on the type of preparation, the catechin dose or the length of the trial ^{5Reference 5Zheng et al. · 2011Meta-analysisGreen tea intake lowers fasting serum total and LDL cholesterol in adults: a meta-analysis of 14 randomised controlled trialsView study →}.

A review describes catechins reducing cholesterol absorption and raising the excretion of sterols and bile acids ^{8Reference 8Moore et al. · 2009ReviewGreen tea (Camellia sinensis) catechins and vascular function — reviewView study →}.

Green, black and herbal teas made from Camellia sinensis all showed antioxidant activity in the laboratory, ranked green above black ^{9Reference 9Chan et al. · 2011In vitroAntioxidant and antibacterial properties of green, black, and herbal teas of Camellia sinensis — in vitroView study →}.

In a placebo-controlled crossover trial, 18 healthy volunteers drank 150 mL of 1% green tea twice daily for four weeks. Their white blood cells resisted DNA damage from an oxidant better, and oxidative DNA damage fell by about 20%. A urine marker of whole-body oxidative stress did not change ^{7Reference 7Han et al. · 2011Clinical trialGenoprotective effects of green tea (Camellia sinensis) in human subjects: results of a controlled supplementation trialView study →}.

Only a small share of catechins reaches the blood. After tea or tea extract, about 7% of the catechins are excreted in urine as metabolites, and about 40% are broken down in the colon ^{49Reference 49EFSA Panel on Food Additives et al. · 2018ReviewScientific opinion on the safety of green tea catechins — regulatory reviewView study →}.

In 66 patients with coronary artery disease, black tea improved flow-mediated dilation of the brachial artery both 2 hours after 450 mL and after four weeks of 900 mL a day, while water did not. A matching dose of caffeine alone had no short-term effect ^{11Reference 11Duffy et al. · 2001RCTShort- and long-term black tea consumption reverses endothelial dysfunction in patients with coronary artery disease — randomised controlled trialView study →}.

In 30 healthy male smokers, green-tea drinks with 580 mg of catechins a day for two weeks improved forearm blood flow and raised plasma nitric oxide, while 80 mg did not ^{10Reference 10Oyama et al. · 2010RCTGreen tea catechins improve human forearm endothelial dysfunction and have antiatherosclerotic effects in smokers — randomised controlled trialView study →}.

In the UK Biobank (498,043 adults, followed for a median 11.2 years), people who drank two or more cups a day had about 9–13% lower all-cause mortality than non-drinkers ^{12Reference 12Inoue-Choi et al. · 2022ObservationalTea consumption and all-cause and cause-specific mortality in the UK Biobank — prospective cohort studyView study →}. A 2024 meta-analysis of 38 cohort datasets (1.96 million people) found cardiovascular mortality about 14% lower at the highest intakes ^{13Reference 13Kim et al. · 2024Meta-analysisTea consumption and risk of all-cause, cardiovascular disease, and cancer mortality: a meta-analysis of thirty-eight prospective cohort data setsView study →}. In two Shanghai cohorts, green tea was linked to lower cardiovascular mortality (HR 0.86) ^{14Reference 14Zhao et al. · 2017ObservationalGreen tea consumption and cause-specific mortality: results from two prospective cohort studies in ChinaView study →}.

Gap: Mortality and outcome data are observational — confounded by overall lifestyle — and endothelial RCTs are short-term surrogate-marker studies, not hard-endpoint trials.

The clearest cognitive effects come from L-theanine combined with caffeine. In 27 healthy volunteers, 100 mg of L-theanine with 50 mg of caffeine improved the speed and accuracy of attention-switching at 60 minutes and reduced distraction in a memory task. Caffeine alone improved alertness and attention-switching accuracy ^{16Reference 16Owen et al. · 2008RCTThe combined effects of L-theanine and caffeine on cognitive performance and mood — randomised controlled trialView study →}.

A 2025 meta-analysis of randomised trials found small-to-moderate benefits of the combination on cognition and mood in the first two hours after intake. The confidence intervals were often wide ^{15Reference 15Payne et al. · 2025Meta-analysisEffects of tea (Camellia sinensis) or its bioactive compounds L-theanine or L-theanine plus caffeine on cognition, sleep, and mood in healthy participants: a systematic review and meta-analysis of randomised controlled trialsView study →}.

A systematic review of 21 studies concluded that green tea’s effects on cognition come from caffeine and L-theanine together, with less effect from either alone ^{17Reference 17Mancini et al. · 2017Systematic reviewGreen tea effects on cognition, mood and human brain function: a systematic reviewView study →}.

The benefits are short-lived.

A review describes EGCG suppressing NF-κB signalling and inflammatory cytokines, and other catechins acting on the NADPH oxidases behind oxidative stress ^{8Reference 8Moore et al. · 2009ReviewGreen tea (Camellia sinensis) catechins and vascular function — reviewView study →}.

In smokers, high-dose green-tea catechins lowered C-reactive protein and other markers of inflammation and oxidative stress ^{10Reference 10Oyama et al. · 2010RCTGreen tea catechins improve human forearm endothelial dysfunction and have antiatherosclerotic effects in smokers — randomised controlled trialView study →}. No trial has tested tea as a treatment for an inflammatory disease.

A polyphenol-rich hot-water extract of tea flowers reduced acute swelling and immune-driven liver inflammation in mice ^{18Reference 18Chen et al. · 2012AnimalAnti-inflammatory effects of a polyphenols-rich extract from tea (Camellia sinensis) flowers in acute and chronic mice models — animalView study →}.

A meta-analysis found that catechins, or an EGCG and caffeine mixture, reduced body weight and helped maintain weight loss by about 1.3 kg. The effect looked smaller in people with high habitual caffeine intake, but that difference was not statistically significant ^{20Reference 20Hursel et al. · 2009Meta-analysisThe effects of green tea on weight loss and weight maintenance: a meta-analysisView study →}.

A Cochrane review of trials lasting at least 12 weeks found a small, non-significant weight loss outside Japan, unlikely to be clinically important ^{19Reference 19Jurgens et al. · 2012Systematic reviewGreen tea for weight loss and weight maintenance in overweight or obese adults — systematic review (Cochrane)View study →}.

Gap: Effects are marginal and inconsistent between populations; not a reliable weight-loss intervention.

A meta-analysis of 17 randomised trials (1,133 people) found that green tea lowered fasting glucose by 0.09 mmol/L and HbA1c by 0.30%, with lower fasting insulin in the higher-quality trials ^{21Reference 21Liu et al. · 2013Meta-analysisEffect of green tea on glucose control and insulin sensitivity: a meta-analysis of 17 randomised controlled trialsView study →}.

A meta-analysis restricted to people with type 2 diabetes found no effect on fasting glucose, fasting insulin, HbA1c or HOMA-IR ^{22Reference 22Asbaghi et al. · 2021Meta-analysisEffect of green tea on glycemic control in patients with type 2 diabetes mellitus: a systematic review and meta-analysisView study →}.

A review describes EGCG, tannins and theaflavins improving insulin sensitivity in laboratory and animal studies ^{8Reference 8Moore et al. · 2009ReviewGreen tea (Camellia sinensis) catechins and vascular function — reviewView study →}.

Gap: The benefit seen in mixed/healthy populations disappears in the group that matters clinically — established type 2 diabetics — so glycemic use is not supported for treatment.

Catechins inhibit cancer-cell growth in the laboratory ^{23Reference 23Filippini et al. · 2020Systematic reviewGreen tea (Camellia sinensis) for the prevention of cancer — systematic review (Cochrane)View study →}. A 2021 meta-analysis of 113 prospective studies covering 26 cancer sites found little evidence that tea drinking is associated with cancer risk, with weak evidence for a few sites ^{24Reference 24Zhao et al. · 2021Meta-analysisTea drinking and risk of cancer incidence: a meta-analysis of prospective cohort studies and evidence evaluationView study →}.

In Asian cohorts, the highest green-tea intake was linked to lower liver-cancer risk (RR 0.88), but one cup a day was not, and the link held only in women ^{25Reference 25Huang et al. · 2016Meta-analysisGreen tea and liver cancer risk: a meta-analysis of prospective cohort studies in Asian populationsView study →}.

A Cochrane review concluded that the evidence from trials and observational studies is inconsistent and gives limited support for a protective effect ^{23Reference 23Filippini et al. · 2020Systematic reviewGreen tea (Camellia sinensis) for the prevention of cancer — systematic review (Cochrane)View study →}.

Gap: Cohort findings are confounded (smoking, diet, hot-beverage temperature) and site-specific; no RCT demonstrates cancer prevention, and Cochrane rates the evidence low quality.

In a four-week crossover trial in 30 healthy adults, 200 mg a day of L-theanine lowered scores for depression, trait anxiety and poor sleep ^{27Reference 27Hidese et al. · 2019RCTEffects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomised controlled trialView study →}.

In an 8-week double-blind trial in 46 people with generalised anxiety disorder, L-theanine (450–900 mg a day) added to their antidepressant did not beat placebo for anxiety or insomnia severity. Sleep satisfaction was rated better on L-theanine ^{26Reference 26Sarris et al. · 2019RCTL-theanine in the adjunctive treatment of generalised anxiety disorder: a double-blind, randomised, placebo-controlled trialView study →}.

Gap: Positive results are in non-clinical, low-stress populations on subjective scales; the one rigorous clinical-anxiety trial was null.

In the laboratory, hot-water extracts of green tea inhibited three gram-positive bacteria. Black tea inhibited two of them, and none of the teas were active against gram-negative bacteria ^{9Reference 9Chan et al. · 2011In vitroAntioxidant and antibacterial properties of green, black, and herbal teas of Camellia sinensis — in vitroView study →}.

In another laboratory study, tea extract was more active against multidrug-resistant Salmonella Typhi than against susceptible strains, alone and combined with the antibiotic nalidixic acid ^{28Reference 28Farooqui et al. · 2015In vitroSynergistic antimicrobial activity of Camellia sinensis and Juglans regia against multidrug-resistant bacteria — in vitroView study →}.

There are no animal-infection or clinical data.

In rats with a 6-OHDA model of Parkinson’s disease, a standardised green-tea extract and the catechins epicatechin and EGCG improved movement and memory and reversed oxidative stress in the striatum. The authors attributed this to their antioxidant and anti-inflammatory activity ^{29Reference 29Bitu Pinto et al. · 2015Neuroprotective properties of the standardised extract from Camellia sinensis (green tea) in the 6-OHDA model of Parkinson’s disease — animalView study →}.

There is no human trial in any neurodegenerative disease.

In rats, a hot-water infusion of Sri Lankan black tea produced mild, dose-dependent diuresis with raised sodium output. The effect tracked the tea’s caffeine content, and the study pointed to aldosterone inhibition, carbonic-anhydrase inhibition and a thiazide-like action ^{30Reference 30Abeywickrama et al. · 2010AnimalOral diuretic activity of hot water infusion of Sri Lankan black tea (Camellia sinensis L.) in rats — animalView study →}.

Gap: One rat study only, with no human data.

Caffeine. Tea’s caffeine can cause sleeplessness, jitteriness, palpitations and a brief rise in blood pressure in sensitive people. EFSA considers single doses up to 200 mg and daily intakes up to 400 mg safe for healthy adults, and notes that 100 mg close to bedtime can affect sleep in some people ^{50Reference 50EFSA Panel on Dietetic Products et al. · 2015ReviewScientific opinion on the safety of caffeine — regulatory reviewView study →}. A cup of black tea contains about 50 mg of caffeine, against about 90 mg for a cup of filter coffee ^{50Reference 50EFSA Panel on Dietetic Products et al. · 2015ReviewScientific opinion on the safety of caffeine — regulatory reviewView study →}.

Green-tea extracts and the liver. EFSA considers the catechins in traditionally brewed green tea safe at the amounts people usually drink, though rare, probably idiosyncratic cases of liver injury have followed green tea infusions ^{49Reference 49EFSA Panel on Food Additives et al. · 2018ReviewScientific opinion on the safety of green tea catechins — regulatory reviewView study →}. In trials, extracts providing 800 mg or more of EGCG a day raised liver enzymes compared with placebo ^{49Reference 49EFSA Panel on Food Additives et al. · 2018ReviewScientific opinion on the safety of green tea catechins — regulatory reviewView study →}. Case reports link liver injury to EGCG intakes from 140 mg to about 1,000 mg a day, with large differences in individual susceptibility ^{31Reference 31Oketch-Rabah et al. · 2020ReviewUnited States Pharmacopeia (USP) comprehensive review of the hepatotoxicity of green tea extractsView study →}. In a year-long trial of 843 mg of EGCG a day, one genetic variant was linked to larger rises in the liver enzyme ALT ^{32Reference 32Acosta et al. · 2023RCTHepatotoxicity with high-dose green tea extract: effect of catechol-O-methyltransferase and UGT1A4 genotypes — randomised controlled trialView study →}. One report describes impending acute liver failure in a teenage boy taking a weight-loss product containing green-tea extract ^{33Reference 33Patel et al. · 2013Case reportGreen tea extract: a potential cause of acute liver failure — case reportView study →}.

Taking an extract as a single dose on an empty stomach raises EGCG levels in the blood, and USP labelling advises taking it with food ^{31,49Reference 31Oketch-Rabah et al. · 2020ReviewUnited States Pharmacopeia (USP) comprehensive review of the hepatotoxicity of green tea extractsView study →Reference 49EFSA Panel on Food Additives et al. · 2018ReviewScientific opinion on the safety of green tea catechins — regulatory reviewView study →}.

Interactions. Drinking 700 ml of green tea a day for two weeks cut blood levels of the beta-blocker nadolol by 85% and blunted its effect on blood pressure in healthy volunteers ^{54Reference 54Misaka et al. · 2014Clinical trialGreen tea ingestion greatly reduces plasma concentrations of nadolol in healthy subjects — clinical studyView study →}. The likely cause is that tea blocks OATP1A2, a transporter that carries nadolol in from the gut ^{54Reference 54Misaka et al. · 2014Clinical trialGreen tea ingestion greatly reduces plasma concentrations of nadolol in healthy subjects — clinical studyView study →}. In volunteers, green tea catechins also lowered digoxin levels by about a third ^{56Reference 56Kim et al. · 2018Clinical trialEffect of green tea catechins on the pharmacokinetics of digoxin in humans — clinical studyView study →}. Effects on warfarin and simvastatin have been small ^{35Reference 35Werba et al. · 2015ReviewOverview of green tea interaction with cardiovascular drugs — reviewView study →}.

Green or black tea taken with folic acid cut peak blood folate by about 39% ^{37Reference 37Alemdaroglu et al. · 2008RCTInfluence of green and black tea on folic acid pharmacokinetics in healthy volunteers: potential risk of diminished folic acid bioavailability — randomised crossoverView study →}.

Drug metabolism. Catechins partly inhibit several CYP enzymes in the laboratory ^{34Reference 34Teschke et al. · 2014ReviewGreen tea extract and the risk of drug-induced liver injury — reviewView study →}. In 42 healthy volunteers, four weeks of green tea catechins left CYP1A2, CYP2C9 and CYP2D6 activity unchanged and had only a small effect on a CYP3A4 drug, judged not clinically significant ^{55Reference 55Chow et al. · 2006Clinical trialEffects of repeated green tea catechin administration on human cytochrome P450 activity — clinical studyView study →}.

Bortezomib. EGCG blocked the anticancer drug bortezomib in mice only at blood levels far above those reached from tea or supplements. The authors concluded that people on bortezomib need not avoid green tea or EGCG supplements ^{36Reference 36Bannerman et al. · 2011Preclinical evaluation of the antitumor activity of bortezomib in combination with vitamin C or with epigallocatechin gallate, a component of green tea — animalView study →}.

Very hot drinks. The International Agency for Research on Cancer classes drinking any beverage very hot, above 65 °C, as probably carcinogenic to the oesophagus, based on studies where tea or maté is drunk at about 70 °C ^{53Reference 53International Agency for Research on Can · 2016ReviewIARC Monographs evaluate drinking coffee, maté, and very hot beverages — Press Release NoView study →}. Letting tea cool a little before drinking avoids this.

Fluoride. A woman who drank about two gallons of extra-strong instant tea a day from the age of 12 developed skeletal fluorosis ^{38Reference 38Whyte et al. · 2008Case reportSkeletal fluorosis from instant tea — case reportView study →}.