L-theanine, an amino acid found in tea plants, is responsible for the unique flavor of green tea. This compound is said to promote relaxation and modify the stimulating effects of the caffeine present in green tea.
Some research has suggested that L-theanine has calming effects. In one study, young women were given either water, 50 mg of L-theanine, or 200 mg of L-theanine once a week. Afterward, their brain waves were measured. Those who received the highest dose (200 mg) produced more alpha waves, an indication of increased relaxation, about 40 minutes after getting the L-theanine. Results of this study were published in the June 1999 issue of Trends in Food Science & Technology. However, more research will be needed to confirm the findings.
Other studies, including animal studies, have suggested that L-theanine has an anti-cancer effect, may enhance learning in animals, and lowers blood pressure in hypertensive rats. But none of these benefits have been demonstrated in humans, at least not yet.
As far as combating anxiety is concerned, I recommend breathing exercises, which are the most effective and fastest-acting method I know. Aerobic exercise is also helpful, and I would recommend avoiding caffeine because its stimulating effects can heighten anxiety.
You also might try tincture of passionflower (Passiflora incarnata), which is mildly relaxing. The dose is one dropperful in a little water, up to four times a day, as needed. Valerian (Valeriana officinalis) is more powerful - use a capsule of the standardized extract only as needed.
For the calming and relaxing benefits of L-theanine, I recommend drinking green tea in moderation rather than taking a supplement. If you find the tea too stimulating, look for a decaffeinated form.
Andrew Weil, M.D.