L-theanine, an FDA approved safe food supplement, is isolated from the tea plant or synthesized artificially and sold in a purified form.

L-theanine reduces cortisol levels and increases relaxation associated with recovery from a stressful task. Importantly, it increases activity in alpha brain waves which improves cognition and enhance mental alertness without any drowsiness. Not only does it helps maintain a calm alertness during the day but also results in deeper sleep at night without morning drowsiness or impaired concentration.

Additionally, L-theanine increases dopamine and serotonin production and decreases norepinephrine concentrations (a stress hormone which affects parts of the brain, where attention and responses are controlled).

Stress Buster
A 2006 study in the "Journal of Herbal Pharmacotherapy," found increased amount of the inhibitory transmitter GABA present in the brain with the use of L-theanine, and therefore concluded its  "calming" effect on the nervous system.

Depression Use
A 2009 study in "Amino Acids" found that the amount of dopamine produced by the brain was increased by L-theanine. Dopamine is a major neurotransmitter that affects mood, attention, memory and sleep. Similarly there are numerous other studies that show its potential in depression treatment.

Schizophrenia Use
According to a study by M.S. Ritsner et al., at the Israel Institute of Technology, Haifa, Israel published in the January 2011 "Journal of Clinical Psychiatry", l-theanine produced modest to moderate improvements in scores on psychopathology evaluation tests done on schizophrenic patients.

Obsessive Compulsive Disorder (OCD)
As obsessive-compulsive disorder often occurs alongside depression or other anxiety disorders which are successfully treated by l-theanine, therefore it has also been used in  treating OCD effectively.

Bipolar Use
Many studies postulate that theanine modulates the levels of neurotransmitters, ie - if one's dopamine levels are too high, theanine lowers it. If one's serotonin levels are too low - it raises it.

Corrects Adrenal Fatigue
With Chronic Stress your adrenal gland can be overworked as it would be producing stress hormones constantly which results in its degradation. The relaxing effect L-theanine has on your body helps to reduce stress levels and restore your adrenal glands to proper function.

High Blood Pressure
Department of experimental psychology at the University of Bristol found that L-theanine significantly reduced the blood pressure elevating properties of caffeine when co-administered with 200 mg of L-theanine. Similarly many other studies show its usage in treating Hypertension.

Immune Booster
A 2008 study in "Nutrition Reviews" showed enhanced action of the immune system cells gamma-delta-T cells with L-theanine.

Anti-Cancer
- An article in the Oct. 1, 2000 issue of "Cancer Letters" explains that l-theanine boosts some chemotherapy drugs' effects. Chemo drugs cause significant decreases in leukocyte white blood cells and bone marrow cells. Theanine significantly reversed these changes, indicating that theanine can selectively moderate chemotherapy toxicity.
- A study published in "Bioscience, Biotechnology and Biochemistry" in April 2002, found beneficial effects against liver cancer with the use of dietary powdered green tea and theanine.

Protection from Glutamate Neurotoxicity
L-theanine also directly provides neuroprotection against glutamate neurotoxicity through blockade of the N-methyl-d-aspartate (NMDA), and a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors.

Safety
The Food and Drug Administration classifies L-theanine supplements as "Generally Recognized as Safe" in dosages up to 250mg. But as always, check with your health care provider before consuming any L-theanine supplement. Do not use L-theanine on children, if you are pregnant or breast feeding.

Dosage
L-Theanine is used is single doses of 50mg to 200mg.

Interactions
Taking theanine along with medications for high blood pressure might cause your blood pressure to go too low.
L-theanine may potentiate other sedative, anxiolytic, or tranquilizing drugs.
L-theanine may also increase the intoxicating effects of alcohol.
There have been no documented interactions of L-theanine with antidepressants, however, you should always consult your health care provider before combining any other drug therapy with L-theanine.

Source: curenature.com