L-theanine is an amino acid. The human body does not produce this compound, and it is not essential for humans. Green tea, black tea, and certain types of mushroom naturally contain L-theanine.L-theanine may affect the levels of certain chemicals in the brain. These include serotonin and dopamine, which influence mood, sleep, and emotion, and cortisol, which helps the body deal with stress.A change in the balance of these chemicals can change a person’s mood or stress levels.Small studies and tests on animals have shown that L-theanine may offer a range of benefits. Experts consider it to be generally safe, but there is currently a lack of high-quality research in humans to confirm its benefits.Better mental focus. Many people drink black or green tea while at work, believing that it boosts their mental focus. Improved sleep.Relaxation.Increased cognitive performance.Weight loss.Boosting the immune system.Reducing blood pressure. Supporting certain cancer drugs.Many people drink black or green tea while at work, believing that it boosts their mental focus. A 2012 studyTrusted Source supports this idea.The researchers found that people who took 100 milligrams (mg) of L-theanine made fewer errors in an attention task than those in the placebo group. Drinking 50 mg of caffeine or combining the L-theanine and caffeine also improved people’s focus.