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Green tea


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.
  • High cholesterol. Taking green tea by mouth seems to lower cholesterol levels. Research suggests that consuming 145-3000 mg of green tea catechins, an antioxidants found in green tea, daily for up to 24 weeks reduces total cholesterol and low-density lipoprotein (LDL or “bad”) cholesterol.
  • Mental alertness. Drinking green tea and other caffeinated beverages seems to help people maintain mental alertness throughout the day. Combining caffeine with sugar as an “energy drink” seems to improve mental performance more than caffeine or sugar alone. Also, taking a combination of green tea extract and L-theanine for seems to improve memory and attention in people with mild mental problems.

Possibly effective for...

  • Abnormal development of cells of the cervix (cervical dysplasia). Taking green tea by mouth or applying it to the skin seems to reduce cervical dysplasia caused by human papilloma virus (HPV) infection.
  • Clogged arteries (coronary artery disease). Population studies suggest that drinking green tea is linked to a reduced risk of clogged arteries. The link seems to be stronger in men than women.
  • Endometrial cancer. Population studies suggest that drinking green tea is linked to a reduced risk of developing endometrial cancer.
  • Low blood pressure. Drinking green tea might help increase blood pressure in elderly people who have low blood pressure after eating.
  • Thick, white patches on the gums (oral leukoplakia). Drinking green tea seems to decrease the size of white patches in people with oral leukoplakia.
  • Osteoporosis. Research suggests that drinking green tea for 10 years is linked to increased bone mineral density. Also, early research suggests that taking a green tea compound containing 500 mg of catechins, an antioxidant in green tea, daily for 24 weeks improves bone strength in post-menopausal women with low bone density.
  • Ovarian cancer. Women who regularly drink tea, including green or black tea, appear to have a lower risk of developing ovarian cancer.
  • Parkinson’s disease. Drinking one to four cups of green tea daily seems to provide the most protection against developing Parkinson’s disease.

Insufficient evidence to rate effectiveness for...

  • Acne. Early research suggests that applying a solution containing a certain chemical found in green tea to the skin for 8 weeks reduces acne.
  • Abnormal protein buildup in the organs (Amyloidosis). Early research suggests that drinking green tea or taking green tea extracts for 12 months protects against an increase in heart mass in people with amyloidosis affecting the heart.
  • Athletic performance. There is inconsistent evidence about the effects of green tea on athletic performance. Some early research suggests that taking green tea extract as a beverage does not improve breathing or performance in people undergoing endurance training. However, other early research suggests that taking seven doses of a certain green tea chemical over three days improves some breathing tests in healthy adults.
  • Bladder cancer, esophadeal cancer, and pancreatic cancer. Most evidence suggests that drinking green tea is linked to a lower risk of bladder, esophageal, and pancreatic cancer. However, there is also some inconsistent evidence that suggests it might not reduce the risk of developing these cancers.
  • Breast cancer. Research suggests that green tea does not seem to reduce the risk of breast cancer in Asian people. However, there is some evidence that it might reduce the risk in Asian-Americans.
  • Heart disease. Population studies suggest that drinking three or more cups of green tea daily is linked to a decreased risk of death from heart disease or any cause.
  • Colds and flu. Early research suggests that taking a specific formulation of green tea and theanine (Thea-flan and Suntheanine) daily for 5 months lowers the risk of developing the flu. Other early research suggests that taking a specific green tea product (ImmuneGuard) reduces cold and flu symptoms and the duration of illness.
  • Colon cancer. Most evidence suggests that drinking green tea does not have any effect on colon cancer risk. However, some research suggests that consuming a high amount is linked to a reduce risk, particularly in women.
  • Diabetes. Research suggests that Japanese adults, particularly women, who drink 6 or more cups of green tea daily, have a lower risk of developing diabetes. However, green tea extract does not seem to help control sugar or insulin levels in people who already have diabetes.
  • Fertility. Early research suggests that taking a specific product containing chasteberry, green tea, L-arginine, vitamins and minerals (FertilityBlend) increases pregnancy rates in women who have trouble conceiving.
  • Stomach cancer. There is inconsistent evidence about the effects of green tea on stomach cancer risk. One study suggests that drinking at least 5 cups of green tea daily does not reduce the risk of stomach cancer. Other research suggests that drinking at least 10 cups of green tea daily reduces the risk of stomach cancer.
  • High blood pressure. There is inconsistent evidence about the effects of green tea on blood pressure. Some research shows that drinking green tea regularly can lower the risk of developing high blood pressure. However, other research shows that it has no effect on blood pressure in people with or without high blood pressure.
  • Allergy to Japanese cedar (pollinosis). Early research suggests that drinking a green tea drink daily for 6 weeks before being exposed to Japanese cedar pollen can reduce allergy symptoms, including throat pain, nose blowing and tears.
  • Leukemia. Population research suggests that Taiwanese people who drink higher amounts of green tea have a lower risk of developing leukemia.
  • Lung cancer. There is inconsistent evidence about the effects of green tea on lung cancer risk. One study suggests that drinking at least 5 cups of green tea daily does not reduce the risk of death related to lung cancer. However, men who consume high amounts of phytoestrogens, chemicals found in green tea, have a lower risk of developing lung cancer.
  • Metabolic syndrome. Early research suggests that taking 1000 mg of green tea extract daily or drinking four cups of green tea daily for 8 weeks does not improve blood pressure, cholesterol levels, or blood sugar in obese people with metabolic syndrome.
  • Obesity. There is inconsistent evidence on the effects of green tea in obese people. Some early research shows that some specific green tea extracts (AR25, Exolise; Sunphenon) reduce weight in people with obesity. Other research suggests that drinking green tea can reduce body weight and body mass index (BMI) in overweight people. However some research suggests that taking green tea extracts or drinking green tea does not reduce body weight or BMI.
  • Mouth cancer. Early research suggests that taking green tea extract three times daily after meals for 12 weeks increases healing responses in people with oral cancer.
  • Gum disease (periodontal disease). Chewing candy that contains green tea extract seems to control plaque build-up on the teeth and reduce gum swelling. Also population research suggests that drinking green tea is linked with a reduced risk of gum disease.
  • Pneumonia. Research suggests that Japanese women who drink green tea have a lower risk of death from pneumonia compared to those who don’t drink green tea.
  • Prostate cancer. Men who drink more green tea or who take products containing green tea antioxidants seem to have a lower risk of developing prostate cancer. However, green tea or green tea extracts do not seem to slow the progression of prostate cancer that has already been diagnosed.
  • Stress. Early research suggests that taking a specific brand of green tea extract (Teavigo) by mouth for 7 days reduces stress and increases calmness.
  • Stroke. According to one study in Japan, drinking 3 cups of green tea daily seems to lower the risk of having a stroke compared to drinking one cup or no tea.
  • Upper respiratory tract infection. Early research suggests that gargling and swallowing green tea (Morgentau) over 4 days is less effective than citrus lozenges (Cystus052) for reducing symptoms of upper respiratory tract infections.
  • Wrinkled skin. Some early research suggests that taking green tea antioxidants twice daily for 2 years does not reduce the signs of sun damage to the face in women. However, applying a green tea cream and taking green tea by mouth daily seems to improve some aspects of skin aging in women.
  • Other conditions.
More evidence is needed to rate green tea for these uses.

How does it work?

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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.

Are there safety concerns?

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Green tea is LIKELY SAFE for most adults when consumed in moderate amounts. Green tea extract is POSSIBLY SAFE for most people when taken by mouth or applied to the skin for a short time. In some people, green tea can cause stomach upset and constipation. Green tea extracts have been reported to cause liver problems in rare cases.

Drinking 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 is LIKELY UNSAFE and 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 – about 2 cups per day – is POSSIBLY SAFE. This amount of green tea provides about 200 mg of caffeine. However, drinking more than 2 cups of green tea per day is POSSIBLY UNSAFE. Consuming more than 2 cups of green tea daily has been linked to an increased risk of miscarriage and other negative effects. Also, caffeine passes into breast milk and can affect a nursing infant. Don’t drink an excessive amount of green tea if you are pregnant or 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 in green tea 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 in green tea 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.

Are there interactions with medications?

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Major

Do not take this combination.

Amphetamines
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.

Cocaine
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.

Ephedrine
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.

Medications moved by pumps in cells (Organic Anion-Transporting Polypeptide Substrates)
Some medications are moved by pumps in cells. Green tea might change how these pumps work and decrease how much of some medications get absorbed by the body. This could make these medications less effective.

Some of these medications that are moved by pumps in cells include bosentan (Tracleer), celiprolol (Celicard, others), etoposide (VePesid), fexofenadine (Allegra), fluoroquinolone antibiotics, glyburide (Micronase, Diabeta), irinotecan (Camptosar), methotrexate, nadolol (Corgard), paclitaxel (Taxol), saquinavir (Fortovase, Invirase), rifampin, statins, talinolol, torsemide (Demadex), troglitazone, and valsartan (Diovan).

Nadolol (Corgard)
Green tea might decrease how much nadolol (Corgard) the body absorbs. Taking green tea along with nadolol (Corgard) might decrease the effectiveness of nadolol (Corgard).

Moderate

Be cautious with this combination.

Adenosine (Adenocard)
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)
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.

Cimetidine (Tagamet)
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.

Clozapine (Clozaril)
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).

Dipyridamole (Persantine)
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.

Disulfiram (Antabuse)
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.

Estrogens
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.

Fluvoxamine (Luvox)
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.

Lithium
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 for depression (MAOIs)
Caffeine in green tea can stimulate the body. Some medications used for depression can also stimulate the body. Taking green tea that contains caffeine along with some medications for depression might cause serious side effects including fast heartbeat, high blood pressure, nervousness, and others.

Some of these medications used for depression include phenelzine (Nardil), tranylcypromine (Parnate), and others.

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).

Nicotine
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.

Pentobarbital (Nembutal)
The stimulant effects of the caffeine in green tea can block the sleep-producing effects of pentobarbital (Nembutal).

Phenylpropanolamine
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.

Riluzole (Rilutek)
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.

Stimulant drugs
Stimulant drugs speed up the nervous system and can make you feel jittery and speed up your heartbeat. Green tea contains caffeine, which can 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.

Some stimulant drugs include diethylpropion (Tenuate), epinephrine, phentermine (Ionamin), pseudoephedrine (Sudafed), and many others.

Theophylline
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)
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.

Minor

Be watchful with this combination.

Alcohol
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.

Fluconazole (Diflucan)
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 changed by the liver (Cytochrome P450 3A4 (CYP3A4) substrates)
Some medications are changed and broken down by the liver. Green tea might decrease how quickly the liver breaks down some medications. Taking green tea while taking some medications that are broken down by the liver can increase the effects and side effects of some medications. However, it is not known if this is a big concern. Talk to your healthcare provider if you are taking any medications that are changed by the liver.

Some medications changed by the liver include lovastatin (Mevacor), ketoconazole (Nizoral), itraconazole (Sporanox), fexofenadine (Allegra), triazolam (Halcion), and many others.

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.

Mexiletine (Mexitil)
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.

Midazolam (Versed)
The body breaks down midazolam (Versed) to get rid of it. Green tea might decrease how quickly the body breaks down midazolam (Versed). Taking green tea along with midazolam (Versed) might increase the effects and side effects of midazolam (Versed). However, it is not known if this is a big concern. Talk with you healthcare provider if you are taking midazolam.

Terbinafine (Lamisil)
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.

Are there interactions with herbs and supplements?

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Bitter orange
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.

Calcium
Green tea contains caffeine. High doses of caffeine can lead to loss of too much calcium in the urine.

Creatine
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.

Folic acid
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 may slow blood clotting (Anticoagulant/Antiplatelet herbs and supplements)
Green tea contains caffeine. Caffeine may slow blood clotting. Using green tea along with other herbs and supplements that might also slow blood clotting could increase the risk of bleeding in some people. Some of these herbs include angelica, clove, danshen, garlic, ginger, ginkgo, Panax ginseng, and others.

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.

Iron
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.

Magnesium
Green tea contains caffeine. Caffeine can increase how quickly the body releases magnesium in the urine.

Are there interactions with foods?

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Iron
Green tea appears to reduce absorption of iron from foods.

Milk
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.

What dose is used?

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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.
  • 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.
APPLIED TO THE SKIN:
  • 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.

Other names

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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.

Methodology

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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).

References

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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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Page last updated: 10 December 2014