Approximately 2.5 million tons of tea are grown and produced worldwide on an annual basis. Written records date the use of the plant as a beverage since at least the tenth century B.C. in China, and it is thought to be close to 5,000 years old. Tea is the most consumed beverage worldwide (after water). It is also one of the most popular herbal infusions in existence—drunk regularly by over half the world population.

The polyphenols in green tea that act as antioxidants may actually inhibit the growth of existing cancer cells. In some animal studies, injections of tea extracts reduced the size of cancerous tumors in animals. The active agent that is thought to have this effect is an antioxidant, epigallocatechin-3-gallate (EGCG).

Recent clinical studies have also indicated that regular use of green tea may reduce the risk of certain types of cancer, including oral, skin, prostate, colon, stomach, and rectal. In one clinical trial, patients with pre-cancerous mouth lesions who were treated with green and black tea extracts achieved a 38% decrease in the number of pre-cancerous cells. Late in 2001, researchers acknowledged one reason for green tea's anticancer effect, but further human studies are needed to clearly define its role in cancer prevention.
The antioxidants in green tea may also be helpful in lowering cholesterol and preventing hardening of the arteries and ischemic heart disease. Low flavonoid intake has been linked to atherosclerosis in several studies. The data from one 1999 study, which followed more than 3,400 tea-drinking residents of Rotterdam, the Netherlands, concluded that regular, long-term tea consumption can have a protective effect against severe atherosclerosis.

Another preliminary study published in 1999 in the American Journal of Clinical Nutrition found that green tea extract may increase energy levels and promote fat oxidation, and consequently, may be a useful tool in weight control. A recent study, reported on in early 2002, showed that topically applied green tea extracts can reduce harmful effects of radiation from the sun. Further study might show that green tea polyphenol applications can help prevent sunburns.

In addition to polyphenols, green tea contains several minerals, including fluoride and aluminum. The fluoride in green tea may be useful in fighting tooth decay. Green tea is also an antibacterial agent, and can help to prevent gingivitis and periodontal disease by killing E. coli and streptococcus bacteria. This antibacterial action can also be effective in treating halitosis, or bad breath, by killing odor-causing bacteria.

As an herbal remedy, green tea is often recommended to ease stomach discomfort, vomiting, and to stop diarrhea. The antibacterial action of tea is useful in treating infections and wounds.