L-theanine is found mostly in teas and is considered an amino acid. It has shown positive implication of improving attention and the amount of perceived stress on the body. Although L-theanine is considered a relaxing agent when taken with caffeine studies have shown strong synergistic effects.(40) A 2008 study showed that when theanine was paired with caffeine it was further able to increase performance and reaction time compared to caffeine alone. Studies have also linked theanine to decreasing the amount of headaches associated  with stimulants such as caffeine.(41)(42) L-theanine has positive benefits on cardiovascular health as it helps with regulating nitric oxide (NO).(40)