Green tea has been used for thousands of years in Asia as both a beverage and an herbal medicine. This herbal tea contains catechin, which is a type of tannin that acts as an astringent. Research suggests that men and women in Japan who drink five to six cups of green tea each day have much lower rates of cancer than people who do not. Green tea is also thought to lower cholesterol and blood sugar, control high blood pressure, stop tooth decay, and fight viruses. Green tea has even been credited with the power to slow down the aging process.