What is it?
Green tea is a product made from the Camellia sinensis plant. It can be prepared as a beverage, which can have some health effects. Or an “extract” can be made from the leaves to use as medicine.
Green tea is used to improve mental alertness and thinking.
It is also used for weight loss and to treat stomach disorders, vomiting, diarrhea, headaches, bone loss (osteoporosis), and solid tumor cancers.
Some people use green tea to prevent various cancers, including breast cancer, prostate cancer, colon cancer, gastric cancer, lung cancer, solid tumor cancers and skin cancer related to exposure to sunlight. Some women use green tea to fight human papilloma virus (HPV), which can cause genital warts, the growth of abnormal cells in the cervix (cervical dysplasia), and cervical cancer.
Green tea is also used for Crohn’s disease, Parkinson’s disease, diseases of the heart and blood vessels, diabetes, low blood pressure, chronic fatigue syndrome (CFS), dental cavities (caries), kidney stones, and skin damage.
Instead of drinking green tea, some people apply green tea bags to their skin to soothe sunburn and prevent skin cancer due to sun exposure. Green tea bags are also used to decrease puffiness under the eyes, as a compress for tired eyes or headache, and to stop gums from bleeding after a tooth is pulled.
Green tea in candy is used for gum disease.
Green tea is used in an ointment for genital warts. Do not confuse green tea with oolong tea or black tea. Oolong tea and black tea are made from the same plant leaves used to make green tea, but they are prepared differently and have different medicinal effects. Green tea is not fermented at all. Oolong tea is partially fermented, and black tea is fully fermented.
How effective is it?
Natural Medicines Comprehensive Database rates effectiveness based on scientific evidence according to the following scale: Effective, Likely Effective, Possibly Effective, Possibly Ineffective, Likely Ineffective, Ineffective, and Insufficient Evidence to Rate.
The effectiveness ratings for GREEN TEA are as follows:
Likely effective for...
- Genital warts. A specific green tea extract ointment (Veregen, Bradley Pharmaceuticals) is FDA-approved for treating genital warts.
- Increasing mental alertness, due to the caffeine content of green tea.
Possibly effective for...
- Preventing dizziness upon standing up (orthostatic hypotension) in older people.
- Preventing bladder, esophageal, ovarian, and pancreatic cancers. Women who regularly drink tea, including green tea or black tea, appear to have a significantly lower risk of developing ovarian cancer compared to women who never or seldom drink tea. In one study, women who drank 2 or more cups of green tea each day had a 46% lower risk of getting ovarian cancer than women who didn’t drink green tea.
- Reducing the risk or delaying the onset of Parkinson’s disease. Drinking one to four cups of green tea daily seems to provide the most protection against developing Parkinson’s.
- Low blood pressure. Green tea might help in elderly people who have low blood pressure after eating.
- Decreasing high levels of fat, like cholesterol and triglycerides, in the blood (hyperlipidemia).
- Reducing abnormal development and growth of cells of the cervix (cervical dysplasia) caused by human papilloma virus (HPV) infection.
Possibly ineffective for...
Insufficient evidence to rate effectiveness for...
- Weight loss. Taking a specific green tea extract (EGCG) seems to help moderately overweight people lose weight. But green tea doesn’t help people keep the weight off.
- High blood pressure. Some research shows that drinking green tea regularly seems to lower the chance of getting high blood pressure. But not all research agrees.
- Stroke prevention. According to a large study done in Japan, drinking 3 cups of green tea per day seems to significantly lower the risk of having a stroke, compared to drinking one cup or no tea. Women seem to benefit more than men.
- Weak bones (osteoporosis). Population research suggests that drinking green tea for 10 years is associated with stronger bones.
- Type 2 diabetes. Drinking green tea may help prevent diabetes. Research suggests that Japanese adults who drink 6 or more cups per day of green tea have a 33% lower risk of developing type 2 diabetes compared to those who drink one cup per day or less. This is especially true for women.
- Breast cancer. Green tea does not seem to prevent breast cancer in Asian populations. However, in Asian-American populations, some evidence suggests that drinking green tea might reduce the risk of developing breast cancer. Most research on green tea for breast cancer has been in Asian populations. The effect of green tea on breast cancer risk in Western populations is less clear.
- Gum disease (gingivitis). Chewing candy that contains green tea extract seems to control plaque build-up on the teeth and reduce gum swelling.
- Prostate cancer. Chinese men who drink more green tea seem to have a lower risk of developing prostate cancer. The more tea they drink, the more their risk drops.
- Chronic fatigue syndrome (CFS).
- Heart disease prevention.
- Kidney stones.
- Lung cancer.
- Stomach cancer.
- Skin cancer.
- Dental cavities.
- Cervical cancer.
- Gastric cancer.
- Other conditions.
More evidence is needed to rate green tea for these uses.
The useful parts of green tea are the leaf bud, leaf, and stem. Green tea is not fermented and is produced by steaming fresh leaves at high temperatures. During this process, it is able to maintain important molecules called polyphenols, which seem to be responsible for many of the benefits of green tea.
Polyphenols might be able to prevent inflammation and swelling, protect cartilage between the bones, and lessen joint degeneration. They also seem to be able to fight human papilloma virus (HPV) infections and reduce the growth of abnormal cells in the cervix (cervical dysplasia). Research cannot yet explain how this works.
Green tea contains 2% to 4% caffeine, which affects thinking and alertness, increases urine output, and may improve the function of brain messengers important in Parkinson’s disease. Caffeine is thought to stimulate the nervous system, heart, and muscles by increasing the release of certain chemicals in the brain called “neurotransmitters.”
Antioxidants and other substances in green tea might help protect the heart and blood vessels.
Green tea is LIKELY SAFE for most adults. Green tea extract is POSSIBLY SAFE for most people for short-term use. In some people, green tea can cause stomach upset and constipation. Green tea extracts have been reported to cause liver problems in rare cases.
Too much green tea — more than five cups per day, for example — is POSSIBLY UNSAFE. It can cause side effects because of the caffeine. These side effects can range from mild to serious and include headache, nervousness, sleep problems, vomiting, diarrhea, irritability, irregular heartbeat, tremor, heartburn, dizziness, ringing in the ears, convulsions, and confusion. Green tea seems to reduce the absorption of iron from food. Drinking very high doses of green tea can actually be fatal. The fatal dose of caffeine in green tea is estimated to be 10-14 grams (150-200 mg per kilogram). Serious toxicity can occur at lower doses.
Caffeine is POSSIBLY SAFE in children in amounts commonly found in foods.
Green tea interacts with many medications, as explained below.
Special precautions & warnings:
Pregnancy and breast-feeding: If you are pregnant or breast-feeding, green tea in small amounts is POSSIBLY SAFE. Do not drink more than 2 cups a day of green tea. This amount of tea provides about 200 mg of caffeine. Consuming more than this amount has been linked to an increased risk of miscarriage and other negative effects. Caffeine passes into breast milk and can affect a nursing infant. Don’t drink an excessive amount of green tea if you are breast-feeding.
“Tired blood” (anemia): Drinking green tea may make anemia worse.
Anxiety disorders: The caffeine in green tea might make anxiety worse.
Bleeding disorders: Caffeine might increase the risk of bleeding. Don’t drink green tea if you have a bleeding disorder.
Heart conditions: Caffeine in green tea might cause irregular heartbeat.
Diabetes: Caffeine might affect blood sugar control. If you drink green tea and have diabetes, monitor your blood sugar carefully.
Diarrhea. Green tea contains caffeine. The caffeine in green tea, especially when taken in large amounts, can worsen diarrhea.
Irritable bowel syndrome (IBS): Green tea contains caffeine. The caffeine in green tea, especially when taken in large amounts, can worsen diarrhea and might worsen symptoms of IBS.
Glaucoma: Drinking green tea increases pressure inside the eye. The increase occurs within 30 minutes and lasts for at least 90 minutes.
High blood pressure: The caffeine in green tea might increase blood pressure in people with high blood pressure. However, this does not seem to occur in people who regularly drink green tea or other products that contain caffeine.
Liver disease: Green tea extract supplements have been linked to several cases of liver damage. Green tea extracts might make liver disease worse.
Weak bones (osteoporosis): Drinking green tea can increase the amount of calcium that is flushed out in the urine. Caffeine should be limited to less than 300 mg per day (approximately 2-3 cups of green tea). It is possible to make up for some calcium loss caused by caffeine by taking calcium supplements.
Do not take this combination.
Stimulant drugs such as amphetamines speed up the nervous system. By speeding up the nervous system, stimulant medications can make you feel jittery and increase your heart rate. The caffeine in green tea might also speed up the nervous system. Taking green tea along with stimulant drugs might cause serious problems including increased heart rate and high blood pressure. Avoid taking stimulant drugs along with caffeine.
Stimulant drugs such as cocaine speed up the nervous system. By speeding up the nervous system, stimulant medications can make you feel jittery and increase your heart rate. The caffeine in green tea might also speed up the nervous system. Taking green tea along with stimulant drugs might cause serious problems including increased heart rate and high blood pressure. Avoid taking stimulant drugs along with caffeine.
Stimulant drugs speed up the nervous system. Caffeine (contained in green tea) and ephedrine are both stimulant drugs. Taking green tea along with ephedrine might cause too much stimulation and sometimes serious side effects and heart problems. Do not take caffeine-containing products and ephedrine at the same time.
Be cautious with this combination.
Green tea contains caffeine. The caffeine in green tea might block the effects of adenosine (Adenocard). Adenosine (Adenocard) is often used by doctors to do a test on the heart, called a cardiac stress test. Stop consuming green tea or other caffeine-containing products at least 24 hours before a cardiac stress test.
Antibiotics (Quinolone antibiotics)
The body breaks down caffeine to get rid of it. Some antibiotics might decrease how quickly the body breaks down caffeine. Taking these antibiotics along with green tea can increase the risk of side effects including jitteriness, headache, increased heart rate, and other side effects.
Some antibiotics that decrease how quickly the body breaks down caffeine include ciprofloxacin (Cipro), enoxacin (Penetrex), norfloxacin (Chibroxin, Noroxin), sparfloxacin (Zagam), trovafloxacin (Trovan), and grepafloxacin (Raxar).
Birth control pills (Contraceptive drugs)
The body breaks down the caffeine in green tea to get rid of it. Birth control pills can decrease how quickly the body breaks down caffeine. Taking green tea along with birth control pills can cause jitteriness, headache, fast heartbeat, and other side effects.
Some birth control pills include ethinyl estradiol and levonorgestrel (Triphasil), ethinyl estradiol and norethindrone (Ortho-Novum 1/35, Ortho-Novum 7/7/7), and others.
Bortezomib (Velcade) is used in certain types of cancers. Green tea might interact with bortezomib (Velcade) and decrease its effectiveness for treating certain types of cancer. If you take bortezomib (Velcade) avoid taking green tea products.
Green tea contains caffeine. The body breaks down caffeine to get rid of it. Cimetidine (Tagamet) can decrease how quickly your body breaks down caffeine. Taking cimetidine (Tagamet) along with green tea might increase the chance of caffeine side effects including jitteriness, headache, fast heartbeat, and others.
The body breaks down clozapine (Clozaril) to get rid of it. The caffeine in green tea seems to decrease how quickly the body breaks down clozapine (Clozaril). Taking green tea along with clozapine (Clozaril) can increase the effects and side effects of clozapine (Clozaril).
Green tea contains caffeine. The caffeine in green tea might block the affects of dipyridamole (Persantine). Dipyridamole (Persantine) is often used by doctors to do a test on the heart called a cardiac stress test. Stop drinking green tea or other caffeine-containing products at least 24 hours before a cardiac stress test.
The body breaks down caffeine to get rid of it. Disulfiram (Antabuse) can decrease how quickly the body gets rid of caffeine. Taking green tea (which contains caffeine) along with disulfiram (Antabuse) might increase the effects and side effects of caffeine, including jitteriness, hyperactivity, irritability, and others.
The body breaks down the caffeine in green tea to get rid of it. Estrogens can decrease how quickly the body breaks down caffeine. Taking estrogen pills and drinking green tea can cause jitteriness, headache, fast heartbeat, and other side effects. If you take estrogen pills, limit your caffeine intake.
Some estrogen pills include conjugated equine estrogens (Premarin), ethinyl estradiol, estradiol, and others.
The body breaks down the caffeine in green tea to get rid of it. Fluvoxamine (Luvox) can decrease how quickly the body breaks down caffeine. Taking green tea along with fluvoxamine (Luvox) might cause too much caffeine in the body, and increase the effects and side effects of caffeine.
Your body naturally gets rid of lithium. The caffeine in green tea can increase how quickly your body gets rid of lithium. If you take products that contain caffeine and you take lithium, stop taking caffeine products slowly. Stopping caffeine too quickly can increase the side effects of lithium.
Medications for asthma (Beta-adrenergic agonists)
Green tea contains caffeine. Caffeine can stimulate the heart. Some medications for asthma can also stimulate the heart. Taking caffeine with some medications for asthma might cause too much stimulation and cause heart problems.
Some medications for asthma include albuterol (Proventil, Ventolin, Volmax), metaproterenol (Alupent), terbutaline (Bricanyl, Brethine), and isoproterenol (Isuprel).
Medications that can harm the liver (Hepatotoxic drugs)
Green tea extracts might harm the liver. Taking green tea extracts along with medication that might also harm the liver can increase the risk of liver damage. Do not take green tea extracts if you are taking a medication that can harm the liver.
Some medications that can harm the liver include acetaminophen (Tylenol and others), amiodarone (Cordarone), carbamazepine (Tegretol), isoniazid (INH), methotrexate (Rheumatrex), methyldopa (Aldomet), fluconazole (Diflucan), itraconazole (Sporanox), erythromycin (Erythrocin, Ilosone, others), phenytoin (Dilantin) , lovastatin (Mevacor), pravastatin (Pravachol), simvastatin (Zocor), and many others.
Medications that slow blood clotting (Anticoagulant / Antiplatelet drugs)
Green tea might slow blood clotting. Taking green tea along with medications that also slow clotting might increase the chances of bruising and bleeding.
Some medications that slow blood clotting include ardeparin (Normiflo), aspirin, clopidogrel (Plavix), diclofenac (Voltaren, Cataflam, others), dipyridamole (Persantine), ibuprofen (Advil, Motrin, others), naproxen (Anaprox, Naprosyn, others), dalteparin (Fragmin), enoxaparin (Lovenox), heparin, ticlopidine (Ticlid), warfarin (Coumadin), and others.
Medications used to treat cancer (Boronic acid-based proteasome inhibitors)
Green tea might interact with some medications used to treat cancer (boronic acid-based proteasome inhibitors). This might decrease its effectiveness of these medications for treating certain types of cancer. If you take one of these medications for cancer, avoid taking green tea products. Some of these drugs include bortezomib (Velcade).
Stimulant drugs such as nicotine speed up the nervous system. By speeding up the nervous system, stimulant medications can make you feel jittery and increase your heart rate. The caffeine in green tea might also speed up the nervous system. Taking green tea along with stimulant drugs might cause serious problems, including increased heart rate and high blood pressure. Avoid taking stimulant drugs along with caffeine.
The stimulant effects of the caffeine in green tea can block the sleep-producing effects of pentobarbital (Nembutal).
Green tea contains caffeine. Caffeine can stimulate the body. Phenylpropanolamine can also stimulate the body. Taking green tea and phenylpropanolamine together might cause too much stimulation and increase heartbeat, increase blood pressure, and cause nervousness.
The body breaks down riluzole (Rilutek) to get rid of it. Drinking green tea can decrease how quickly the body breaks down riluzole (Rilutek) and increase the effects and side effects of riluzole.
Green tea contains caffeine. Caffeine works similarly to theophylline. Caffeine can also decrease how quickly the body gets rid of theophylline. Taking green tea along with theophylline might increase the effects and side effects of theophylline.
Verapamil (Calan, Covera, Isoptin, Verelan)
The body breaks down the caffeine in green tea to get rid of it. Verapamil (Calan, Covera, Isoptin, Verelan) can decrease how quickly the body gets rid of caffeine. Drinking green tea and taking verapamil (Calan, Covera, Isoptin, Verelan) can increase the risk of side effects for caffeine including jitteriness, headache, and an increased heartbeat.
Warfarin (Coumadin) is used to slow blood clotting. Large amounts of green tea have been reported to decrease the effectiveness of warfarin (Coumadin). Decreasing the effectiveness of warfarin (Coumadin) might increase the risk of clotting. It is unclear why this interaction might occur. Be sure to have your blood checked regularly. The dose of your warfarin (Coumadin) might need to be changed.
Be watchful with this combination.
The body breaks down the caffeine in green tea to get rid of it. Alcohol can decrease how quickly the body breaks down caffeine. Taking green tea along with alcohol might cause too much caffeine in the bloodstream and caffeine side effects including jitteriness, headache, and fast heartbeat.
Green tea contains caffeine. The body breaks down caffeine to get rid of it. Fluconazole (Diflucan) might decrease how quickly the body gets rid of caffeine and cause caffeine to stay in the body too long. Taking fluconazole (Diflucan) along with green tea might increase the risk of side effects such as nervousness, anxiety, and insomnia.
Medications for diabetes (Antidiabetes drugs)
Green tea contains caffeine. There is conflicting evidence that caffeine might either increase or decrease blood sugar. Diabetes medications are used to lower blood sugar. Taking some medications for diabetes along with caffeine might decrease the effectiveness of diabetes medications. Monitor your blood sugar closely. The dose of your diabetes medication might need to be changed.
Some medications used for diabetes include glimepiride (Amaryl), glyburide (DiaBeta, Glynase PresTab, Micronase), insulin, pioglitazone (Actos), rosiglitazone (Avandia), chlorpropamide (Diabinese), glipizide (Glucotrol), tolbutamide (Orinase), and others.
Green tea contains caffeine. The body breaks down caffeine to get rid of it. Mexiletine (Mexitil) can decrease how quickly the body breaks down caffeine. Taking Mexiletine (Mexitil) along with green tea might increase the caffeine effects and side effects of green tea.
The body breaks down the caffeine in green tea to get rid of it. Terbinafine (Lamisil) can decrease how fast the body gets rid of caffeine. Taking green tea along with terbinafine (Lamisil) can increase the risk of caffeine side effects including jitteriness, headache, increased heartbeat, and other effects.
Bitter orange, used along with caffeine or caffeine-containing herbs such as green tea, can increase blood pressure and heart rate in otherwise healthy people. This might damage the heart and blood vessels.
Caffeine-containing herbs and supplements
Green tea contains caffeine. Using green tea along with other herbs and supplements that contain caffeine might increase the effects of caffeine, and also its unwanted side effects. Some natural products that contain caffeine include coffee, black tea, oolong tea, guarana, mate, cola, and others.
There is some concern that combining caffeine, ephedra, and creatine might increase the risk of serious unwanted side effects. One athlete who used this combination, as well as some other supplements to improve his performance, suffered a stroke. Researchers worry the stroke might have been caused by the supplements.
Ephedra (Ma Huang)
Don't take green tea with ephedra. The caffeine in green tea might increase the effects of ephedra. Using ephedra with caffeine might increase the risk of serious life-threatening or disabling conditions such as hypertension, heart attack, stroke, seizures, and death.
There is some concern that green tea might decrease the activity of folic acid, leaving the body with less than the amount of folic acid it needs.
Herbs and supplements that might harm the liver
In several cases, people who took green tea developed liver damage. Researchers worry that the damage might have been linked to the green tea. Taking green tea extracts with other herbs or supplements that might harm the liver could increase the risk of harm to the liver. Other products that might adversely affect the liver include bishop's weed, borage, chaparral, uva ursi, and others.
Green tea might reduce the absorption of iron supplements. For most people, this effect will not be enough to make a difference in their health. But people who don't have enough iron in their system would be wise to drink green tea between meals rather than with meals to lessen this interaction.
Green tea appears to reduce absorption of iron from foods.
Adding milk to tea seems to reduce some of tea's benefits for the heart and blood vessels. Milk might bind and prevent absorption of the antioxidants in tea. But this is controversial. More research is needed to find out how important this interaction really is.
The following doses have been studied in scientific research:
AS A DRINK:
Doses of green tea vary significantly, but usually range between 1-10 cups daily. The commonly used dose of green tea is based on the amount typically consumed in Asian countries, which is about 3 cups per day, providing 240-320 mg of the active ingredients, polyphenols. To make tea, people typically use 1 teaspoon of tea leaves in 8 ounces boiling water.
APPLIED TO THE SKIN
- For headache or restoring mental alertness: tea providing is up to 250 mg of caffeine per day, or approximately 3 cups of tea per day.
- For improving thinking: tea providing 60 mg of caffeine, or approximately one cup.
- For reducing cholesterol: drinking 10 or more cups per day has been associated with decreased cholesterol levels. Theaflavin-enriched green tea extract, 375 mg daily for 12 weeks, has also been used for lowering cholesterol.
- For human papilloma virus (HPV) infections of the cervix: green tea extract, 200 mg daily alone or in combination with topical green tea ointment, for 8-12 weeks.
- For preventing Parkinson’s disease:
- Men consuming 421-2716 mg total caffeine (approximately 5-33 cups of green tea) daily have the lowest risk of developing Parkinson’s disease. However, a significantly lower risk is also associated with consumption of as little as 124-208 mg of caffeine (approximately 1-3 cups of green tea) daily.
- In women: more moderate caffeine consumption seems to be best, equivalent to approximately 1-4 cups of green tea per day.
- For human papillomavirus (HPV) infections of the cervix: green tea ointment alone or in combination with oral green tea extract, twice weekly for 8-12 weeks.
- For genital warts: a specific green tea extract ointment (Veregen, Bradley Pharmaceuticals) providing 15% kunecatechins applied three times daily to external warts for up to 16 weeks has been used.
Camellia sinensis, Camellia thea, Camellia theifera, Constituant Polyphénolique de Thé Vert, CPTV, EGCG, Epigallo Catechin Gallate, Épigallo-Catéchine Gallate, Epigallocatechin Gallate, Extrait de Camellia Sinensis, Extrait de Thé, Extrait de Thé Vert, Extrait de Thea Sinensis, Green Sencha Tea, Green Tea Extract, Green Tea Polyphenolic Fraction, GTP, GTPF, Japanese Tea, Kunecatechins, Poly E, Polyphenon E, PTV, Té Verde, Tea, Tea Extract, Tea Green, Thé, Thé de Camillia, Thé Japonais, Thé Vert, Thé Vert de Yame, Thé Vert Sensha, Thea bohea, Thea sinensis, Thea viridis, Yame Green Tea, Yame Tea.
To learn more about how this article was written, please see the Natural Medicines Comprehensive Database methodology.methodology (http://www.nlm.nih.gov/medlineplus/druginfo/natural/methodology.html).
To see all references for the Green tea page, please go to http://www.nlm.nih.gov/medlineplus/druginfo/natural/960.html.
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