Doctor's Best® Suntheanine L-Theanine is a nutritional supplement suitable for vegetarians that contains 100% Suntheanine , a pure natural form of the amino acid L-theanine manufactured in Japan from green tea leaves, to encourage an optimistic attitude, attentiveness and to relieve the stress and tension of everyday pressures. This successful form of theanine has been the focus of several research studies, and it has been shown that one of the green tea’s ingredients, L-theanine (y-glutamylethylamide) contained in the plant's leaves, is responsible for the flavor and soothing effects of green tea. These studies further shown that added L-theanine passes through the blood-brain barrier within 30 minutes of ingestion and can quickly provide its soothing effect on the brain.