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14 Benefits of Green Tea + Side Effects & Risks

Written by Carlos Tello, PhD (Molecular Biology) | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Carlos Tello, PhD (Molecular Biology) | Last updated:

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Green Tea

Green tea contains nutrients and antioxidants that provide many health benefits – from reducing the risk of heart disease, cancer, and diabetes to increasing longevity or improving brain function. Read this article to learn all about this nutritional drink.

What Is Green Tea?

Green tea is a type of tea made from the same plant as black and oolong tea (Camellia sinensis). However, the leaves and buds used to make green tea don’t undergo the same withering and oxidation processes than those employed in these two other tea types.

Green tea contains polyphenols and catechins, which are antioxidant micronutrients naturally found in plants. By reducing free radical production, they may help prevent several diseases.

Epigallocatechin gallate (EGCG) is the main polyphenol found in green tea and has many potential benefits to human health [1].

L-theanine, an amino acid similar to glutamate and glutamine, is another component abundantly found in green tea. It has been specially investigated for its potential to improve cognitive function and curb anxiety [2, 3].

Green tea is also a rich source of minerals [1].

Snapshot

Proponents

  • High content of antioxidant compounds
  • Approved for genital and anal warts
  • May help prevent heart disease
  • May help prevent some cancer types
  • May help prevent diabetes
  • May improve cognitive function
  • May improve oral health
  • May help lose weight

Skeptics

  • Insufficient evidence for some benefits
  • May cause adverse effects associated with caffeine
  • High doses may damage the liver
  • May contain toxic levels of aluminum
  • Lowered testosterone, reduced iron absorption, damaged the pancreas, and enlarged the thyroid gland in animal studies

Health Benefits of Green Tea

Effective for:

Warts

A topical ointment with green tea extract (Polyphenon E, also called Sinecatechins) was approved by the FDA in 2006 for the treatment of genital and anal warts. In 4 clinical trials on over 2,200 people, it was effective and well-tolerated [4, 5, 6, 7].

Likely Effective for:

Preventing Heart Disease

A meta-analysis of 9 studies and almost 260,000 people associated green tea consumption with a reduced incidence of heart disease. It was dose-dependently associated with fewer heart attacks. Drinking over 10 cups/day was associated with lower LDL levels. Its flavanols were identified as the protective compounds in another meta-analysis [8, 9].

This protective effect seems largely due to its ability to lower blood pressure and improve blood fat profile. Multiple meta-analyses of clinical trials found that drinking green tea reduced blood pressure, total cholesterol, and LDL cholesterol [10, 11, 12, 13, 14, 15].

The evidence suggests that green tea may help prevent heart disease by lowering blood pressure and cholesterol (both total and bound to LDL). You may discuss with your doctor if it may help as a complementary measure in your case.

Possibly Effective for:

1) Increasing Longevity

In 2 studies of over 350,000 Japanese elderly people, drinking more than 3 cups of green tea per day was associated with lower rates of death from all causes, heart disease, stroke, and respiratory diseases in both genders and from cancer in women [16, 17].

In another study on over 14,000 Japanese elderly people, drinking green tea was associated with a 76% lower mortality during the 6-year follow-up period of the study. Green tea was associated with reduced death from all causes and heart disease, as well as with a lower incidence of colorectal cancer [18].

Polyphenols in green tea protected telomeres from shortening in cell-based studies, suggesting their potential to increase longevity [19].

All in all, the evidence suggests that green tea may help increase longevity. You may combine it with a healthy lifestyle if your doctor recommends it.

2) Preventing Cancer

Oxidative damage may trigger several cancer types. As a rich source of antioxidants, green tea may help prevent them [20].

Colorectal Cancer

In an observational study on almost 70,000 Chinese women, regular consumption of green tea was associated with a reduced incidence of colorectal cancer [21].

Although they generally concluded that the evidence was insufficient and larger, better-designed clinical studies were needed, 4 meta-analyses associated drinking green tea with a lower incidence of colorectal cancer [22, 23, 24, 25].

In 2 clinical trials on 276 people, supplementation with green tea extract reduced the incidence of a colorectal cancer type (metachronous colorectal adenoma) [26, 27].

Breast Cancer

In a study on over 4,000 French women, consuming green tea and other antioxidant beverages was associated with a lower incidence of breast cancer. However, a study on over 1,500 women failed to associate regular green tea intake with breast cancer [28, 29].

In a small clinical trial on 12 women with breast cancer, a formulation with epigallocatechin gallate efficiently delivered this polyphenol into the blood and tumors and reduced a biomarker of breast cancer (Ki-67) [30].

Meta-analyses found a slight trend towards reducing breast cancer incidence in women drinking high amounts of green tea, although they generally considered the evidence insufficient due to the high heterogeneity of the studies. The most recent meta-analysis did find green tea effective at protecting from breast cancer, especially from its recurrence [31, 32, 33, 34, 35].

Prostate Cancer

In an observational study on almost 50,000 Japanese men, green tea consumption was associated with a reduced frequency of advanced prostate cancer [36].

Two meta-analyses concluded that green but not black tea may help prevent prostate cancer, especially in Asians and in people taking over 7 cups/day. However, another meta-analysis of 21 studies found no association [37, 38, 39].

In a clinical trial on 60 men with a condition that often develops into prostate cancer (high-grade prostate intraepithelial neoplasia), the daily treatment with green tea catechins helped prevent this type of cancer. However, a similar trial on almost 100 men at risk of prostate cancer found green tea ineffective as a preventive measure. Nevertheless, a meta-analysis of 13 studies found green tea slightly effective [40, 41, 42].

Although they reduced the levels of a prostate cancer marker (PSA) in some studies, green tea polyphenols had little or no effectiveness in preventing prostate cancer in most trials on people without high-grade prostate intraepithelial neoplasia. One of them even associated green tea supplements with an increased incidence [43, 44, 45, 46, 47, 48].

Liver Cancer

Green tea polyphenols reduced liver cancer biomarkers in 2 clinical trials on over 200 people at high risk of developing this condition [49, 50]

Three meta-analyses found an association between green tea intake and reduced incidence of primary liver cancer [51, 52, 53].

Lung Cancer

In 2 clinical trials on over 200 heavy smokers, drinking green tea reduced oxidative damage to the DNA, possibly reducing the risk of lung cancer [54, 55].

Three meta-analyses concluded that green tea may protect against lung cancer [56, 57, 58].

However, a small trial on 17 people with advanced lung cancer found green tea ineffective as a chemotherapeutic agent [59].

Ovarian Cancer

Three meta-analyses associated green tea consumption with a lower incidence of ovarian cancer [60, 61, 62].

However, a clinical trial on 46 women with advanced ovarian cancer found epigallocatechin gallate inefficient as a chemotherapeutic agent [63].

Mouth Cancer

In a clinical trial on 59 people with mouth lesions that may develop into mouth cancer (oral leukoplakia), drinking green tea and applying it topically reduced the size of the lesions and some markers of oral cancer progression [64].

A meta-analysis of 14 articles and over 4,600 people associated green but not black tea consumption with a reduced incidence of mouth cancer [65].

Leukemia

In 3 pilot trials on 87 people with leukemia, green tea extract reduced lymphocyte counts [66, 67, 68].

Other Cancer Types

Studies investigating the potential preventive role of green tea on the following cancer types produced mixed results that don’t allow us to draw conclusions:

To sum up, limited evidence suggests that green tea may help prevent some cancer types such as colorectal, breast, lung, ovarian, and prostate cancer. However, the majority of studies covered in this section deal with associations only, which means that a cause-and-effect relationship hasn’t been established. Other dietary, environmental, and genetic factors may have influenced the incidence of cancer.

You may take green tea as a preventive measure if your doctor recommends it. Importantly, the fact that green tea consumption was associated with a reduced incidence of a cancer type by no means implies that it may help fight it. Never use green tea or any other supplements to replace approved anticancer medication.

3) Teeth Health

Mouth rinses with green tea extract reduced plaque buildup, gum inflammation, and the salivary counts of microbes that may cause cavities and gum disease (Streptococcus mutans and lactobacilli) in clinical trials on 225 people [82, 83, 84].

Green tea extract also inhibiting cavities-causing bacteria (S. mutans and S. salivarius) in test tubes. Similarly, epigallocatechin gallate reduced the growth of S. mutans when incorporated into a dental glue [85, 86].

Mouth rinses with green tea reduced dentin wear in 2 small trials on 22 volunteers. Chewing gum with green tea ever remineralized enamel lesions in another trial on 45 people [87, 88, 89].

A green tea gel applied after a non-surgical treatment reduced gum bleeding and inflammation in a clinical trial on 48 people. Similarly, a mouthwash with green tea reduced pain after tooth removal in another trial on 48 people [90, 91].

Oral green tea tablets reduced the compounds that cause bad breath (volatile sulfur-containing compounds) in a clinical trial on 54 people [92].

The existing evidence suggests that green tea may improve oral health by preventing infections, protecting the enamel, and reducing inflammation and bad breath. You may discuss with your doctor if it may be helpful as an add-on to your treatment regime. Importantly, never take green tea in place of what your doctor recommends or prescribes.

4) Cognitive Function

Epigallocatechin gallate, caffeine, and L-theanine all may work to improve brain function and health.

The amount of caffeine found in green tea is enough to help improve memory and reaction time, as well as to boost dopamine and norepinephrine in the brain [93].

In a clinical trial on 12 healthy volunteers, green tea extract improved brain activity in a key region to working memory processing (dorsolateral prefrontal cortex). Green tea increased brain activity, especially theta waves, in another study on 8 healthy volunteers. This suggests its potential to improve alertness and attention. L-theanine especially increased alpha waves, associated with mental alertness and arousal, in another trial on 35 healthy people [94, 95, 96].

A combination of green tea extract and L-theanine improved memory and attention in a clinical trial on 92 people with mild brain damage [97].

In another trial on 27 elderly people with cognitive dysfunction, drinking green tea didn’t improve their symptoms but reduced oxidative damage, possibly helping reduce their progression [98].

Epigallocatechin gallate improved visual recognition memory, working memory, inhibitory control, and adaptive behavior in 2 clinical trials on over 100 people with Down’s syndrome. It also improved cognitive function in mice with this condition [99, 100].

To sum up, limited evidence suggests that green tea may improve cognitive function in both healthy people and those with cognitive dysfunction. You may discuss with your doctor if it may be helpful in your case.

5) Diabetes

In a meta-analysis of 7 studies and over 286,000 people, drinking green tea was associated with an 18% lower incidence of type 2 diabetes [101].

Its potential to improve blood sugar control in people with type 2 diabetes or at risk of developing this condition is less clear. While it was effective at reducing fasting blood sugar in 2 meta-analyses (and fasting insulin at high doses in one of them), a third one found green tea ineffective at lowering type 2 diabetes markers in people at risk [102, 103, 104].

The differences observed may be due to the heterogeneity of the populations. A meta-analysis found green tea especially effective at lowering fasting blood sugar in people below 55 years old or of Asian origin [105].

Taken together, the evidence suggests that green tea may help prevent diabetes, being its potential to lower blood fasting sugar less clear. You may take it for this purpose if your doctor determines that it may help in your case. Never take green tea instead of the antidiabetic medication prescribed by your doctor.

6) Weight Loss

The effects of green tea on weight loss have been tested in multiple clinical trials. Although several studies found that it helped, the results were modest and mixed [106, 107, 108, 109, 110].

Three meta-analyses of 44 studies found that green tea and its catechins promoted a slight weight loss [111, 112, 113].

A meta-analysis of 8 studies found that they may do so by promoting fat burning and energy expenditure. However, 2 other meta-analyses of 21 studies concluded that green tea catechins only promote fat burning in combination with caffeine [114, 115, 116].

Green tea had no effect on the appetite hormones leptin and ghrelin in a meta-analysis of 11 trials [117].

Although the results were modest, most studies concluded that green tea (especially if it contains caffeine) may help lose weight. However, doing more exercise and improving your diet may be safer and more effective for this purpose. Discuss with your doctor if you may take green tea to lose weight and carefully follow their recommendations.

Insufficient Evidence for:

1) Skin Aging and UV Damage

The topical application of creams with green tea extract improved skin elasticity, reduced wrinkles, and protected from the harmful effects of UV radiation (aging, burns, immunosuppression) in 5 clinical trials on 87 people. However, another trial on 40 women found it ineffective [118, 119, 120, 121, 122].

The results of trials using oral green tea polyphenols are mixed. These compounds had similar effects to topical preparations in 4 clinical trials on over 150 people but were ineffective in 3 trials on over 150 people [123, 124, 125, 126, 127, 128, 129].

Due to the mixed results, it’s difficult to draw conclusions on the effects of oral and topical green tea on skin aging. Further clinical research is needed to shed some light on this potential use.

2) Acne

In 2 clinical trials on 99 people with acne, drinking decaffeinated green tea daily for 4 weeks reduced inflammatory lesions on the nose, chin, and around the mouth [130, 131].

Cell-based studies found that epigallocatechin gallate can block fat secretion in the skin and inhibit a microbe that causes acne (Propionibacterium acnes) [131].

Although the results are promising, the evidence is insufficient to claim for certain that green tea helps with acne. More clinical trials on larger populations are warranted.

3) Infections

In a clinical trial on almost 400 people, topical green tea resolved skin MRSA infections in all cases [132].

In a clinical trial on 200 healthcare workers for the elderly, taking green tea catechins (378 mg/day) and theanine (210 mg/day) reduced the incidence of common flu. However, gargling with green tea was ineffective for flu prevention in another trial on over 750 children [133, 134].

Green tea was also of little against upper respiratory infections in a clinical trial on almost 300 people [134, 135].

In test tubes, green tea and its components inhibited several infectious bacteria [136, 137].

A few clinical trials with mixed results and some cell-based studies cannot be considered sufficient evidence that green tea helps fight infections. Larger, more robust clinical trials are needed to back this potential benefit.

4) Bone Loss

In a clinical trial on 171 postmenopausal women with low bone density, an intervention with green tea polyphenols and tai chi improved bone formation biomarkers and muscle strength, while reducing oxidative damage that may worsen bone loss. The intervention was safe and well-tolerated [138, 139, 140].

However, another study on almost 1,000 postmenopausal women given green tea extract for 12 months found it ineffective to improve bone mineral density [141].

Epigallocatechin gallate promoted bone development in cell-based studies [142].

Two studies with opposite results are clearly insufficient to support this potential health benefit. More research is needed to shed some light on it.

5) Liver Health

In a clinical trial on 17 people with non-alcoholic fatty liver disease, high-catechin green tea reduced fat content, inflammation, and oxidative damage in the liver [143].

Similarly, it reversed mild fatty liver in another trial on 60 people with high blood cholesterol [144].

However, high doses (1315 mg catechins/day) for 12 months were toxic to the liver in a study on over 1,000 women [145].

Two small clinical trials cannot be considered sufficient evidence that green tea improves fatty liver disease. Additionally, high catechin doses have been clearly associated with liver damage.

6) Hair Loss

The polyphenols in green tea stimulated human hair growth in test tubes and on the scalps of 2 volunteers. Further research should replicate this preliminary finding [146].

Side Effects & Precautions

This list does not cover all possible side effects. Contact your doctor or pharmacist if you notice any other side effects.

Call your doctor for medical advice about side effects. In the US, you may report side effects to the FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada, you may report side effects to Health Canada at 1-866-234-2345.

Drinking too much green tea may cause the adverse effects associated with caffeine intake. These include anxiety, accelerated heart rate, and sleep disturbances. L-theanine found in green tea can in turn cause dizziness, headaches, and digestive problems [147, 148].

Due to the effects of caffeine on heart rate, people suffering from heart conditions, as well as pregnant and breastfeeding women, should drink green tea in moderation.

Because caffeine has diuretic effects, people on diuretics should drink green tea in moderation.

Some studies have revealed the capacity of tea plants to accumulate high levels of aluminum. This heavy metal may build up in the body and cause brain and kidney damage. To reduce your potential exposure to aluminum, make sure to obtain green tea from reputable sources [149, 150].

Although rare, excessive epigallocatechin gallate (over 400 mg/day) can cause liver toxicity and even liver failure [151, 152, 153].

Tea polyphenols reduced iron absorption in rats. People with anemia may want to cut down on green tea intake [154].

Epigallocatechin gallate acted as a pro-oxidant, rather than an antioxidant, in the pancreatic insulin-secreting cells of diabetic mice. Further clinical studies should determine its safety in people with type 2 diabetes [155].

In rats, green tea consumption lowered testosterone levels by 20% and sperm count by 2%, suggesting its potential to reduce male fertility [156].

At high doses, green tea extract induced thyroid enlargement (goiter) and altered thyroid function in rats. However, people drinking green tea are unlikely to reach these doses [157].

Drug and Supplement Interactions

Supplement/Herb/Nutrient-drug interactions can be dangerous and, in rare cases, even life-threatening. Always consult your doctor before supplementing and let them know about all drugs and supplements you are using or considering.

  • Fish oil: its combination with green tea catechins may increase fat and glucose metabolism [158].
  • Ubiquinone: this supplement increased the antioxidant effects of epigallocatechin gallate in rats [159].
  • Curcumin: its combination with epigallocatechin gallate reduced cancer cell growth [160].
  • Capsicum vanilloids: found in hot peppers, capsicum vanilloids help kill cancer cells. Their combination with green tea had synergistic effects [161].
  • Vitamin E: its combination with green tea polyphenols improved exercise capacity, lowered glucose levels, and reduced body weight in humans [162].

Supplementation

Dosage

An ointment with 15% green tea extract is approved for genital and anal warts [4, 5, 6, 7].

Green tea or supplements with its active compounds are not approved for any other uses, but studies saw benefits from drinking 1-10 cups per day. Discuss with your doctor if green tea may help you as a complementary approach and what the appropriate dose should be in your case.

Do not exceed 400 mg of epigallocatechin gallate per day, as more has been associated with liver issues.

Increasing the Availability of EGCG

Epigallocatechin gallate generally has poor bioavailability. You can increase the uptake of this polyphenol by taking it in combination with [163]:

Because calcium and magnesium reduce its absorption, you may want to avoid combining it with supplements or foods high in these nutrients.

About the Author

Carlos Tello

Carlos Tello

PhD (Molecular Biology)
Carlos received his PhD and MS from the Universidad de Sevilla.
Carlos spent 9 years in the laboratory investigating mineral transport in plants. He then started working as a freelancer, mainly in science writing, editing, and consulting. Carlos is passionate about learning the mechanisms behind biological processes and communicating science to both academic and non-academic audiences. He strongly believes that scientific literacy is crucial to maintain a healthy lifestyle and avoid falling for scams.

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