Green, black, and oolong teas all come from the same plant, camellia sinensis, but are prepared using different methods. To produce green tea, fresh leaves from the plant are lightly steamed.
Tea has been used for medicinal purposes in China and Japan for thousands of years.
Current uses of green tea as a beverage or dietary supplement include improving mental alertness, relieving digestive symptoms and headaches, and promoting weight loss. Green tea and its extracts, such as one of its components, EGCG, have been studied for their possible protective effects against heart disease and cancer.
Green tea is consumed as a beverage. It is also sold in liquid extracts, capsules, and tablets and is sometimes used in topical products (intended to be applied to the skin).
How much do we know?
Although many studies have been done on green tea and its extracts, definite conclusions cannot yet be reached on whether green tea is helpful for most of the purposes for which it is used.
What have we learned?
There’s evidence that green tea enhances mental alertness, as would be expected because of its caffeine content.
The U.S. Food and Drug Administration has approved a specific green tea extract ointment as a prescription drug for treating genital warts.
Studies of green tea and cancer in people have had inconsistent results. The National Cancer Institute does not recommend for or against using green tea to reduce the risk of any type of cancer.
Very few long-term studies have investigated the effects of tea on heart disease risk. However, the limited evidence currently available suggests that both green and black tea might have beneficial effects on some heart disease risk factors, including blood pressure and cholesterol.
Green tea extracts haven’t been shown to produce a meaningful weight loss in overweight or obese adults. They also haven’t been shown to help people maintain a weight loss.
The National Center for Complementary and Integrative Health (NCCIH) is funding research on green tea and its extracts, including studies of the effects of high doses of tea components on the liver, whether substances in green tea can be helpful for iron overload disease, and the safety of a component of green tea in people who are HIV-positive.
What do we know about safety?
Green tea, when consumed as a beverage, is believed to be safe when used in moderate amounts.
Liver problems have been reported in a small number of people who took concentrated green tea extracts. Although the evidence that the green tea products caused the liver problems is not conclusive, experts suggest that concentrated green tea extracts be taken with food and that people discontinue use and consult a health care provider if they have a liver disorder or develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice.
Except for decaffeinated green tea products, green tea and green tea extracts contain substantial amounts of caffeine. Too much caffeine can make people feel jittery and shaky; interfere with sleep; and cause headaches.
Green tea has been shown to reduce blood levels (and therefore the effectiveness) of the drug nadolol, a beta-blocker used for high blood pressure and heart problems. It may also interact with other medicines.
Tell all your health care providers about any complementary or integrative health approaches you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.
Source: Herbs at a Glance. https://nccih.nih.gov/health/greentea.