Green tea is a drink made from the unfermented leaves of the Asian plant Camellia sinensis. There is not enough reliable evidence to say that it might prevent certain cancers.
- Green tea is made from the unfermented leaves of the plant Camellia sinensis
- Manufacturers promote it as a dietary supplement and claim that it helps control blood sugar, cholesterol, blood pressure and weight
- There is not enough reliable evidence that it might prevent certain cancers
- Green tea can have side effects
What is green tea?
China and Japan have used green tea as a medicine for many years. The tea is made from the steamed, unfermented leaves of the Camellia sinensis plant. It is also called Chinese tea.
Manufacturers promote it as a dietary supplement. They claim that it helps control:
- blood sugar
- blood pressure
People also drink it because they believe it might prevent certain types of cancer.
The substance in green tea that researchers think is most helpful is a catechin. It is called epigallocatechin-3-gallate (EGCG). It also contains caffeine and theanine.
Why people with cancer use it
There is no real evidence that green tea can help treat cancer.
There is some evidence from early studies to suggest that having green tea might reduce the risk of some cancers. But at the moment the evidence is not strong enough to know this for sure.
People with cancer also drink green tea because they believe it might:
- boost their immune system which might help them 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 no other conventional treatment can
The media has also promoted black tea 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.
How you have it
Green tea, like other teas on the market, comes as ready-made tea bags or leaves. You add boiling water to it.
There are different views on how many cups you should drink each day to get a possible benefit. Manufacturers often suggest having between 3 and 5 cups a day.
Most green teas contain 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. It is uncertain whether removing the caffeine could reduce the possible antioxidant effects.
EGCG, the helpful substance in green tea, is available as green tea extract. People take it as a supplement in liquids, tablets or capsules.
Manufacturers sell green tea as a herbal supplement in most countries. So, manufacturers don’t have to prove that they are safe or even that they have any health benefits.
This means that the product or dose might not have been through tests to see if it works. Keep this in mind when buying supplements.
Green tea is generally safe to drink, but it contains caffeine which is a stimulant. If you take it in large amounts, it can cause you to:
- have difficulty sleeping
- have headaches
- feel jittery or shaky
EGCG in green tea, when taken in large quantities, can cause sickness and an upset stomach in some people. Large amounts can also be toxic to the liver and cause a change to liver enzymes.
Using green tea safely
As green tea is an herbal product it might not have had the necessary tests. These tests check 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 it may change the way some drugs work.
A laboratory study in 2013 showed that EGCG might increase the actions of some anti-cancer drugs. But this substance may also reduce the effect of other medications.
Speak to your doctor before taking large amounts of green tea.
Research into green tea for cancer
Some laboratory studies have shown that extracts from green tea can stop cancer cells from growing.
Green tea contains substances called polyphenols. A sub group of these polyphenols is called catechins. Scientists think catechins give green tea it’s antioxidant properties. The catechin in green tea is called epigallocatechin-3-gallate (EGCG). Researchers think that EGCG has an anti-tumour(anti-cancer) effect.
These lab results are encouraging. But we need evidence from human studies to prove them. The evidence at the moment from human studies is mixed.
In 2020, a Cochrane review looked into green tea being used for the prevention of cancer. Overall there were 1,100,000 participants across all of the studies that were looked into. Overall, the evidence showed that the effects of having green tea to lower cancer risk were inconsistent. They did find a slight improvement in quality of life for people who took green tea.
Researchers published an overview of studies (a
Other studies have shown it may reduce the risk of developing:
- lung cancer
- bladder cancer
- cancer of the food pipe (oesophagus).
In 2018 researchers did another review and meta analysis of several studies. It showed that green tea might help to prevent breast cancer. And that it might especially help to prevent breast cancer from coming back. But the studies in the review were small. The researchers said that we need more scientific studies looking at greater numbers of people.
The research looking at whether it prevents prostate cancer generally and in pre cancerous prostate cells is mixed with some showing it might and others not showing any evidence. But a review study in 2018 suggests that the substances in green tea in laboratory research and early clinical trials suggests it may be of benefit. They suggest we need more research.
Researchers did a study in 2017 on 39 patients. It was on people at risk of colon and rectal cancer. They found that green tea did not reduce the amount of aberrant crypt foci (ACF). ACF are groups of abnormal tube-like glands in the lining of the colon and rectum. They are early changes in the colon and rectum and can lead to cancer.
We need more evidence from randomised controlled clinical trials. This can help us to know whether green tea has a role in preventing cancer. At the moment it is not possible to draw any firm conclusions because:
- only a small number of studies have been done
- there is a lack of randomised controlled clinical 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
There is very little research into whether green tea or green tea extract can help treat cancer.
In 2012 an American study looked at taking a green tea extract called polyphenon E. They gave it to 42 patients. They all had a type of leukaemia called chronic lymphocytic leukaemia (CLL). The people in the trial were not taking any other kind of treatment.
The researchers found that in a third of people the number of leukaemic cells lessened. Their lymph nodes also shrank. This was a small trial, but the results were promising. We need larger trials before we will know whether green tea or its extracts can help people with cancer.
In another review study in 2018 researchers looked at using green tea and turmeric in people with CLL. They found limited evidence and mixed results that it might work.
Research has shown a possible interaction between green tea and the drug bortezomib (Velcade). This early research suggested that green tea might stop Velcade from working well. But we need much more research to find out if green tea has the same effect on humans.
Speak to your doctor if you are taking Velcade and you want to drink green tea. You might have to avoid green tea or the extract EGCG. Some people take EGCG as a supplement in liquid or capsules.
How much it costs
You can find green tea in health food shops, supermarkets, chemists and over the internet. Prices can vary depending on the amount you buy and where you buy it.
The quality or grade of green tea can also vary. Usually, better quality green tea costs more. The best quality tea is made from the first leaf buds that come in the spring.
A word of caution
There are no reports of any other harmful effects from using green tea. Always read the product labels. Check with your doctor to find out about possible interactions. This is when you are taking other drugs, herbs, or supplements.
Do remember that green tea contains caffeine. So, pregnant or breastfeeding mothers should use it with caution.
It is understandable that you might want to try anything if you think it might help treat or cure your cancer. Only you can decide whether to use a complementary cancer therapy such as green tea.
You could harm your health if you stop your cancer treatment for an unproven treatment.
Many websites promote green tea. But no reputable scientific cancer organisations support any of these claims.