Green tea (Chinese tea) | Cancer Research UK
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What green tea is

Green tea is a drink made from the dried leaves of the Asian plant Camellia sinensis. This tea is drunk widely across Asia. The rates of many cancers are much lower in Asia than other parts of the world. Some people believe this is because of the high intake of green tea. 

You may also hear green tea called Chinese tea. The substance in green tea that researchers think is most helpful is called epigallocatechin-3-gallate (EGCG). EGCG is available as green tea extract which some people take as a supplement in liquid or capsules.


Why some people with cancer use green tea

Although there is no real evidence that green tea can help with cancer, some people take it because they believe it might

  • Boost their immune system and fight their cancer
  • Improve health, energy levels and well being
  • Get rid of toxins in the body
  • Give them some control over their cancer and its treatment
  • Treat their cancer if conventional treatment can no longer offer a possible cure

People also use green tea to try and reduce their risk of getting cancer. There is some evidence from early studies of green tea to suggest that it may reduce the risk of some cancers. But at the moment the evidence is not strong enough to know this for sure.

More recently black tea has also been promoted as an anti cancer agent. Black tea comes from the same plant as green tea. But black tea is made from the fermented leaves of the plant.

You can read about why people with cancer use complementary therapies.


Research into green tea and preventing cancer

Some laboratory studies have shown that extracts from green tea can stop cancer cells from growing. Green tea contains substances called polyphenols, and a sub group called catechins, which scientists think give it antioxidant properties. But while these lab results are encouraging, we need evidence from human studies to prove them.

The evidence we have at the moment from human studies is mixed. An overview of studies (a meta analysis) published in 2014 suggested that green tea could reduce the risk of developing mouth cancers. Another meta analysis has suggested that green tea consumption could reduce the risk of lung cancer. One study has shown a protective effect of green tea on bladder cancer development. The risk of developing cancer of the food pipe also seems to be reduced in women who drink green tea.

In July 2009, the Cochrane Library published a review of 51 studies which included more than 1.5 million people. This review was looking for a link between drinking green tea and having a lower risk of cancer. The cancers included in the review were digestive tract cancers, breast cancer, bladder cancer, prostate cancer and lung cancer, as well as cancer of the mouth. The researchers said that that the research so far is conflicting, which means that green tea remains unproven in preventing cancer. The researchers say it is safe to drink green tea in moderate amounts, of 3 to 5 cups a day. You can read the Cochrane review into green tea and preventing cancer on the Cochrane Library website.

In 2008 the Centre for Evidence Based Chinese Medicine in China published a review of 43 studies, 4 randomised controlled trials and 1 meta analysis. The trials looked at studies testing whether green tea can reduce the risk of cancer. The overall quality of these studies was good or moderate. Some trials seemed to suggest that green tea can reduce the risk of developing cancers of the digestive system but other trials did not. 

Other studies have also given conflicting results, for example, a meta analysis published in 2006 suggested that green tea may lower the risk of breast cancer. But an earlier meta analysis could not find any significant evidence to support a reduction in the risk of breast cancer.

It has been suggested that green tea may help to prevent prostate cancer. However, a study of almost 20,000 Japanese men, published in the British Journal of Cancer in 2006, found no relationship between green tea and prostate cancer.

We really need more evidence from randomised controlled clinical trials to help us know whether green tea has a role in preventing cancer. At the moment it is impossible to draw any firm conclusions because

  • Only a small number of studies have been done
  • There is a lack of randomised controlled trials
  • There are differences in lifestyle factors within studies (most studies have been done in East Asia)
  • Different amounts of tea have been taken within the studies
  • There is possible interaction with other treatments or diet supplements used by people in the studies

Research into green tea and treating cancer

Very little research has been done into whether green tea of green tea extract can be helpful in treating cancer. But in 2012 an American study looked at giving a green tea extract called polyphenon E to 42 patients with a type of leukaemia called chronic lymphocytic leukaemia. The people in the trial were not taking any other kind of treatment. 

The researchers found that a third of the patients had a reduction in the number of leukaemic cells and their lymph nodes shrank. This is a small trial but the results are promising. We will need larger trials before we know whether green tea or its extracts can help people with cancer.


Taking green tea

Like most teas on the market, green tea comes as ready made tea bags or leaves that you add to boiling water. You use one tea bag or 1 to 2 teaspoons of the dried leaves per cup.

There are conflicting views on how many cups you should drink each day to get a possible benefit. Manufacturers often suggest taking between 3 and 5 cups a day. Remember that most green tea contains caffeine so it will act as a stimulant and keep you awake if you drink it before going to bed. Some manufacturers sell decaffeinated green tea. But we don’t know whether removing the caffeine could reduce the possible antioxidant effects.

You can also buy green tea as capsules. Because green tea is sold as a herbal supplement in most countries, manufacturers don’t have to prove that they are safe or even have any health benefits. Labels on these supplements usually recommend that you take 3 capsules a day. But this does not necessarily mean the product or dose has been fully tested. So our advice is to buy with caution.


Side effects

Although generally thought to be safe to drink, green tea can cause sickness and an upset stomach in some people. Because green tea contains caffeine it is a stimulant and if taken in large amounts can cause

  • Difficulty sleeping
  • Irritability
  • Anxiety
  • Passing urine often

Using green tea safely

As green tea is a herbal product it hasn't necessarily been thoroughly tested for interactions with foods, medicines or other supplements. This means that it is difficult to know for sure how safe it is. Like any other herbal product or drug, it may affect how you absorb medicines and may change the way some drugs work. A laboratory study in 2013 showed that the substance in green tea called epigallocatechin-3-gallate (EGCG) may increase the actions of some anti cancer drugs. But this substance may also reduce the effect of other drugs.

Recent research has raised concerns about a possible interaction between green tea and the drug bortezomib (Velcade). Animal research has suggested that green tea may stop Velcade from working so well. Do remember that we'd need much more research to find out if green tea has the same effect in humans. But if you are taking Velcade, you may want to avoid green tea or the extract EGCG, which some people take as a supplement in liquid or capsules. If you have any worries, talk it over with your cancer specialist.

There are no reports of any other harmful effects from using green tea. Our advice is to always read the product labels. And if you are taking other drugs, herbs, or supplements, you need to check with your doctor about possible interactions.

It is important to remember that green tea is a source of caffeine. For this reason, pregnant or breast feeding mothers, who are advised to avoid high caffeine doses, should use it with caution.

Always check with your doctor before you start using any type of complementary or alternative therapy. It is very important to find out all you can about the therapy before deciding to use it. We recommend that you don't replace your conventional cancer treatment with any type of supplement like green tea. It may be safe to take it alongside your cancer treatment but check with your doctor first to make sure.

Our message is

  • Be wary of any websites or people who claim that green tea can treat or cure your cancer
  • Be wary of websites selling green tea for a very high price
  • Make sure you look into the information that is available
  • Talk to your cancer doctor before you buy

You may find it useful to read our section on the safety of herbal products and medicines

We also have information about the safety of vitamin and diet supplements.


The cost of green tea

Green tea is sold in health food shops, chemists and over the internet. It is generally quite cheap. But as with many dietary and herbal products, prices vary depending on the amount you buy and where you buy it.

The quality or grade of green tea can also vary. The general rule is that better quality green tea costs more. The best quality tea is made from the first leaf buds that come in the spring. If you buy green tea over the internet, prices can vary even more. For example, one company we looked at online sold 50g of different types of green tea for between £2.30 and £13.50.


Useful organisations

Some of the complementary therapy organisations can offer information about using green tea. 

You can also find detailed information on the CAM-cancer website.

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Updated: 12 January 2015