(See also Overview of Medicinal Herbs and Nutraceuticals.)
Green tea is made from the dried leaves of the same plant as traditional tea. However, traditional tea leaves are fermented, and green tea leaves are steamed but unfermented. Green tea may be brewed and drunk or ingested in tablet or capsule form. It is thought to have effects that protect cells from damage by oxygen, mutations, and cancer. Green tea contains caffeine, but many extracts have been decaffeinated. It is high in flavonoids and catechins.
Green tea is said to have multiple health benefits, but none are supported by strong scientific evidence. People take green tea for many reasons, including prevention of cancer, coronary artery disease, and tooth decay, as well as treatment of external genital warts. Other reasons are protection of skin from the sun, reduction of fat (lipid) levels in the blood, relief of osteoarthritis pain and menopausal symptoms, and enhancement of weight loss, memory, and longevity.
Side effects are related to the effects of caffeine. They include insomnia, anxiety, frequent urination, nausea, diarrhea, irritability, upset stomach, rapid heart rate (tachycardia), and mild tremor. Pregnant women should avoid excessive caffeine. Caffeine in high doses may lead to high blood pressure, delirium, seizures, and irregular heart rhythms.