TL;DR L-theanine is a relatively safe nootropic that relieves stress and anxiety, boosts focus and attention, helps you concentrate and become more creative, and strengthens your immune system. Mix it with a healthy dose of caffeine and you can amplify these effects for optimum performance.
Ever heard of the Limitless movie (or the Netflix show for that matter)? It’s where Bradley Cooper’s character, Edward Morra, found a nootropic drug - NZT-48 - which turned him from a struggling writer into a brain superpower (like 4-digit IQ and memory that remembers practically EVERYTHING).
So first of all - what is “nootropic”?
According to Healthline, nootropics are natural or synthetic substances that can be taken to improve mental performance in healthy people. 
They’re also known as “smart drugs” and “cognitive enhancers” because they take executive functions, memory, creativity, and other cognitive functions to a whole another level. Essentially, nootropics help you gain optimal function for all things brainy!
Second of all - why are we talking about Limitless and nootropics?
Let’s just say that this Limitless drug exists IRL, except that instead of a pill, it’s a naturally-occurring amino acid.
And it also starts with the letter L.
What is L-theanine?
Commonly found in tea leaves and some mushrooms, L-theanine is an amino acid that brings about a trifecta of optimal brain activity:
- It improves cognitive function. L-theanine affect alpha brain waves that bring about alertness sans the jitters, heightened focus, as well as a boost in creativity. In case you’re wondering what alpha brain waves do, they bring about that state of “wakeful relaxation,” the feeling you get when you’re meditating or daydreaming. L-theanine prompts these brain waves to make you feel more relaxed and be more creative.
- L-theanine is a natural anxiolytic.It eases anxiety and brings down high blood pressure. It’s a great stress reliever because it tapers down levels of hormones linked to stress and anxiety... like cortisol. While some anxiolytics such as valerian and hops reduces anxiety through sedation, L-theanine can do just the same without the latter.
- At the same time, it increases the levels of calming hormones in your body. L-theanine boosts the amount of GABA, serotonin, and dopamine - all known to promote relaxation, focus and regulated mood.
The good news doesn’t stop there. L-theanine has been found to contain ZERO trace of toxicity, dependence, and tolerance. This simply means that there’s no need to worry about going overboard - you can take large doses and NOT suffer from any adverse effects.
If you're keen for some L-theanine fix, you can go grab yourself a cup of green or black tea. There are also supplements available in retail or online.
Benefits of L-theanine
If you’re feeling so pumped about L-theanine that you just want to go for a green tea run RIGHT NOW, wait 'til you get to the best part...the health benefits!
Anxiety and stress-relief
Research suggests that L-theanine can help people suffering from schizophrenia and schizoaffective disorders by easing their anxiety. [2, 3]
Studies also show that tea is your best bet in dealing with high anxiety levels.  In fact, a series of five randomized-controlled trials prove that L-theanine reduces stress and anxiety during difficult situations.  Another study shows that our favorite amino acid also promotes relaxation sans drowsiness .
Increased focus, attention, memory and learning
During stressful situations, our body produces stress hormones such as cortisol and corticosterone. These hormones prevent our brains from thinking clearly.
Thanks to L-theanine, which reduces the level of these hormones, we can think properly amidst stress and anxiety. A study conducted in 2013 showed that a combination of L-theanine and caffeine (more of this later) gave a group of young people increased “subjective alertness” and improved cognitive performance as they went through challenging activities. 
Not only can L-theanine reduce anxiety, but it also boosts our focus, attention, memory, and learning. Studies have found that L-theanine enhances these cognitive functions while increasing attention span and reaction time among anxious people. 
Better immune system
Studies have also found that L-theanine strengthens our immune system. Specifically, research showed that it could steer us clear of upper respiratory tract infections.  L-theanine could also ease intestinal tract inflammation. 
Caffeine and L-theanine: the power duo?
Now that we know why L-theanine is called a smart drug, many have vouched for its effects when combined with - wait for it - caffeine.
Apparently, L-theanine has a powerful synergistic relationship with caffeine. Not only is the amino acid just as effective as caffeine when it comes to heightened attention and focus, it also amplifies caffeine’s stimulant properties while getting rid of its infamous downsides - anxiety, increased blood pressure, and jitters.
Here are some of the recent studies saying this is so:
- L-theanine is just as effective as caffeine in boosting cognitive functions and selective attention,according to this 2016 study which compared the effects of high doses of L-theanine against caffeine and a placebo. 
- Caffeine is such a great stimulant that without its side effects (i.e. increased blood pressure) it would’ve been a substance made in heaven, right? Well, this study has proven that L-theanine can eliminate this “vasoconstrictor” effect of caffeine. Not only that, but it can also give you a happier mood and improved behavior. [11, 12]
- While caffeine is known for its several benefits like wakefulness, improved cognitive functions and increased metabolism, it’s also notorious for caffeine-induced anxiety and jitters. Luckily, L-theanine can help get rid of caffeine’s vasoconstrictive effects. At least according to this 2012 study that says it reduces not only increased blood pressure and jitters, but also the anxiety. Sounds like a win-win! 
L-theanine is a relatively safe nootropic that brings about a trifecta of optimal cognitive function. Because of its natural properties, L-theanine helps relieve stress and anxiety, boost focus and attention, helps you concentrate and become more creative, and strengthens your immune system. When mixed with caffeine, you get the positive effects of caffeine - amplified - and without its side effects. If you're sick of caffeine-induced anxiety, jitters, and increased blood pressure, the L-theanine-caffeine power duo is perfect for you!
Here's a video that talks more about L-theanine and what happens when it's combined with caffeine:
 http://apjcn.nhri.org.tw/server/APJCN/17 Suppl 1/167.pdf