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Green Tea

Other common name(s): green tea extract, Chinese tea

Scientific/medical name(s): Camellia sinesis


Green tea is a drink made from the steamed and dried leaves of the Camellia sinesis plant, a shrub native to Asia. Black tea is also made from this plant, but unlike green tea, black tea is made from leaves that have been fermented. Fermentation may reduce the levels of some compounds, such as antioxidants, in the tea.


Some researchers believe green tea may protect against certain types of cancer because it contains antioxidants. Compounds taken from green tea can be useful in humans. One type of green tea extract called sinecatechins is used to make an ointment (Veregen®) that has been tested and approved by the US Food and Drug Administration (FDA) to treat genital warts. Another extract, epigallocatechin gallate (EGCG) is being tested to see if it will help with chronic lymphocytic leukemia.

However, results from human studies of green tea when used as a whole herb have been mixed. More research is needed to find out if green tea can help in any kind of cancer prevention.

How is it promoted for use?

Green tea is widely consumed in Japan, China, and other Asian nations and is becoming more popular in Western nations. Some reports indicate green tea may have the ability to help prevent cancers of the skin, esophagus, stomach, colon, pancreas, lung, bladder, prostate, and breast. A few people suggest that it helps to treat cancer as well.

Green tea contains chemicals known as polyphenols, which have antioxidant properties. The major group of polyphenols in green tea are called catechins, and the most important catechin seems to be epigallocatechin gallate (sometimes called epigallocatechin-3-gallate or EGCG). EGCG may help cause certain types of cancer cells to die in much the same way that normal cells do. This effect is important because cancer cells are different from normal cells in that they do not die when they should—they continue to grow and spread.

Herbalists use green tea and extracts of its leaves for stomach problems, vomiting, and diarrhea and to reduce tooth decay, blood pressure, cholesterol levels, and blockages of the blood vessels in the heart that can lead to heart attacks. Green tea is also promoted as preventing certain bacterial infections. In recent years, some researchers have suggested that black tea may also be effective in cancer prevention. These claims are being studied.

What does it involve?

The typical amount of green tea consumed varies widely, and it is not clear how much might be needed for beneficial effects. Three or more cups a day are typically taken in Asian countries. Green tea is usually brewed using 1 to 2 teaspoons of the dried tea in a cup of hot water, which is then steeped for 3 to 15 minutes. Bottles and cans of prepared green tea are sold under a variety of brand names, often with added ingredients.

Green tea extracts are also available in capsule form. Three capsules of green tea extract a day is a common recommended dosage, but this dosage and its effects remain uncertain. The contents of these extracts vary widely.

What is the history behind it?

The Chinese have been drinking green tea for at least 3,000 years, and it has been popular in some other Asian countries for at least 1,000 years. In recent years, scientists have begun to study its health effects more closely in laboratory and animal studies and in observational human studies.

What is the evidence?

Many laboratory studies have shown green tea acts against cancer cells in cell cultures. Test tube studies have suggested that compounds in the tea may help stop new blood vessels from forming, thereby cutting off the supply of blood to cancer cells. It is tempting to assume that it may therefore help prevent some cancers, but results of studies in humans have been mixed. Most human studies have been epidemiologic studies in East Asia, in which researchers compared tea drinkers with non–tea drinkers while trying to account for other lifestyle differences. It is hard to draw firm conclusions from them unless there are multiple studies in which other factors are ruled out.

Studies looking at large populations in East Asia generally have not found that green tea drinkers have a lower risk of breast, stomach, or colon cancer than non–tea drinkers. One study found that Asian-American women who drink green tea regularly have a lower risk of breast cancer than those who do not. A Chinese study found that green tea drinking was linked to fewer cancers of the esophagus for people who did not smoke. On the other hand, a 2006 Japanese study showed that those with cancer of the esophagus were more likely to be green tea drinkers than those who did not have the cancer. Other studies of green tea’s ability to prevent or treat lung, prostate, bladder, or other types of cancer have yielded similarly mixed results. The FDA reviewed all of the published studies and concluded in early 2011 that it is very unlikely that green tea prevents any type of cancer in humans.

There are also studies looking at green tea extracts such as epigallocatechin gallate (EGCG). One such early (Phase I) study was done at Mayo Clinic to find safe dosing. They gave the extract in pill form to people with early stage chronic lymphocytic leukemia (CLL). These people had no symptoms and would not usually be treated. Some of the people who got EGCG seemed to have fewer cancer cells after taking EGCG than before they started. Eventually the researchers hope to learn if the extract can help people with early stage CLL. Further clinical trials are needed to find out if the EGCG actually changes the course and outcome of their leukemia.

An Italian study published in 2006 looked at men with prostate intraepithelial neoplasi, which sometimes progresses to prostate cancer. The researchers gave half the men green tea extract and the other half sham pills. Over the next year, the men given green tea extract had fewer cases of prostate cancer than the men who got placebos. Other studies looked to see if green tea could help men who already had prostate cancer, but it didn’t seem to reduce the risk of prostate cancer continuing to grow and spread.

While the results of laboratory studies have been promising, at this time the available scientific evidence does not support claims that green tea can help prevent or treat any specific type of cancer in humans. Purified extracts have shown some promise in limited areas. Controlled, randomized clinical trials are needed to learn about the effects of green tea and more about those of its extracts. Several studies are under way.

Are there any possible problems or complications?

This product is sold as a food and/or a dietary supplement in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don't claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

Moderate intake of green tea is generally considered safe. Asians have consumed this tea for thousands of years with few dangerous side effects. However, a few people may have allergic reactions and should stop drinking it should reactions occur.

Drinking large amounts of tea may cause nutritional problems because of the strong binding activities of the polyphenols, which can make it harder for the body to absorb certain medicines and iron supplements. Always tell your doctor and pharmacist about any herbs and supplements you are taking.

Possible effects from too much caffeine are the major concern with green tea. Too much caffeine can lead to irritability, nervousness, nausea, and trouble sleeping. Because caffeine acts as a stimulant, people with irregular heartbeats or who have anxiety attacks should use it cautiously. People who take in large amounts of caffeine as a daily habit can have headaches and other withdrawal symptoms if they stop suddenly.

Women who are pregnant or breast-feeding should not drink green tea in large amounts. Caffeine can cross the placenta and affect the fetus and can also be passed along in breast milk.

Green tea extracts may be unsafe in high doses. Some of the people who got EGCG extracts in clinical trials had nausea, abdominal pain, and lab tests that showed their liver function worsened. High-dose studies in dogs resulted in some deaths, with harm to the liver, intestines, kidneys, and bone marrow. The dogs had fewer bad effects when the EGCG dose was given with food rather than on an empty stomach. Because these studies used standardized extracts and many of the products on the market have varying amounts, it’s hard to guess what dose might cause harm in humans.

Relying on this type of treatment alone and avoiding or delaying conventional medical care for cancer may have serious health consequences.

Additional resources

More information from your American Cancer Society

The following information on complementary and alternative therapies may also be helpful to you. These materials may be found on our Web site (www.cancer.org) or ordered from our toll-free number (1-800-ACS-2345).

Complementary treatments and herbal medicines

Dietary Supplements: What Is Safe?

The ACS Operational Statement on Complementary and Alternative Methods of Cancer Management

Complementary and Alternative Methods and Cancer

Placebo Effect

Finding out about new ways to treat and prevent cancer

Learning About New Ways to Treat Cancer

Learning About New Ways to Prevent Cancer


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Note: This information may not cover all possible claims, uses, actions, precautions, side effects or interactions. It is not intended as medical advice, and should not be relied upon as a substitute for consultation with your doctor, who is familiar with your medical situation.

Last Medical Review: 04/02/2012
Last Revised: 05/04/2012