Green tea has long had many health benefits associated with it and its curative properties have been used in Chinese medicine for thousands of years.

Green tea, black tea and oolong tea are all made with the leaves of the Camellia sinensis bush. Green tea is made from unfermented leaves and is reputed to contain the highest concentration of polyphenols, chemicals that act as powerful antioxidants. The more the leaves are fermented, the lower the polyphenol content and the higher the caffeine content. Tea polyphenols are classified as catechins and green tea contains six primary catechin compounds: catechin, gallaogatechin, epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate (also known as EGCG). EGCG is considered to be the most active component in green tea and is the best researched of all the green tea polyphenols.

Several studies have suggested that green tea and green tea extract have cancer-fighting abilities, possibly because the tea's concentration of antioxidants such as EGCG help minimize the cell damage that can lead to cancer.

A recent study conducted at the Mayo Clinic in Rochester, Minnesota has demonstrated the benefits of green tea in adult sufferers of leukemia. EGCG (the key antioxidant in green tea) has been shown to kill cancer cells taken from leukaemia patients and put in a test tube.

Doctors at the clinic reported that four patients had started using green tea on their own last year, after hearing media reports about its health benefits. All four patients had CLL (chronic lymphocytic leukemia). In the months after they started drinking green tea or taking green tea extracts, three of them showed measurable improvements in their condition. The fourth patient who did not show an improvement in her cancer overall, still exhibited an increase in her white blood cell count.

In one case, a patient had been showing progressive swelling in her lymph nodes before she starting taking green tea capsules twice a day. Swelling of the lymph nodes is one of the characteristic symptoms of CLL. However, over the next year the patients lymph nodes steadily reduced in size.

Other research has come up with an explanation for why regular green tea drinkers have a low incidence of gastric and oesophageal cancers. Researchers have found that EGCG, the anti-cancer compound in green tea, works in a similar way to the anti-cancer drug methotrexate. Both work by blocking the enzyme dihydrofolate reductase, which is essential to the growth of tumour cells. However, the benefit of EGCG over methotrexate is that it has far fewer side effects.

The same study also showed that blocking the dihydrofolate reductase enzyme lowers folic acid levels. This side effect could explain the increased risk of birth defects and explains why some studies suggest green tea increases the risk of birth defects such as spina bifida. However, in a study conducted at Kaiser Permanente Medical Care Program of Northern California in Oakland, researchers found that women who drank more than one half cup of caffeinated green tea every day doubled their odds of conceiving. Women seeking to conceive or who are already pregnant are advised to consult their medical practitioner about the risks associated with drinking green tea.