The short version: L-theanine is a chemical found in tea. It probably decreases some of the actions of glutamate, the main activating neurotransmitter in the brain. Some people take l-theanine along with caffeine in a 1:1 ratio to make the caffeine work better, with less jitters and anxiety. Other times people take it on its own in order to fight anxiety. Usual dose is about 200 – 400 mg once daily. No evidence of significant side effects.

The long version:

1. What kinds of things do people use theanine for?

For anxiety, but in at least four distinct ways:

1. For managing the “jitters” associated with caffeine. Some people take l-theanine with their coffee and say it helps them feel better and clearer-headed than coffee alone.

2. For breakthrough anxiety, ie when someone is feeling especially anxious during a crisis and wants to take something right away to feel better for a few hours.

3. For chronic anxiety, ie when someone who is anxious all the time wants a pill they can take every day.

4. For sleep, especialy sleep problems associated with anxiety.

2. What’s the evidence that theanine works?

I can find one good systematic review of theanine. It looked at nine studies, including eight double-blind RCTs and one single-blind RCT. Some of the studies did physiological tests of stress (eg check the heart rate or the level of stress hormones in the saliva), while others asked people to rate their subjective stress levels. Theanine seemed to do well on several physiologic measures and in a few subjective studies, but did not separate from placebo in the largest study, an eight week trial on subjective anxiety levels. The authors concluded:

The supplementation of [l-theanine] in its pure form at dosages between 200–400 mg/day may help reduce stress and anxiety acutely in people undergoing acute stressful situations, but there is insufficient evidence to suggest it assists in the reduction of stress levels in people with chronic conditions. However, the results of this study suggest that l-theanine taken during times of heightened acute stress or by individuals with a high propensity for anxiety and stress may exhibit beneficial properties via the increased production of alpha waves and decrease of glutamate in the brain.

I think their “insufficient evidence for chronic conditions” conclusion comes from a combination of two studies: Sarris (2019) and Ritsner (2009). Sarris found that l-theanine even at very high doses was not helpful in generalized anxiety disorder (though possibly people seemed to sleep better). Ritsner found it did seem to help schizophrenics become less anxious over two months.

A few articles look directly at the caffeine/l-theanine combination, for example this one (ignore their conclusions section and look at the actual statistics), which finds l-theanine, when compared to caffeine alone, makes users calmer, less jittery, and more alert. This study adds some interesting physiological findings; it “did not lead to a positive impact on behavior”, but only used about a quarter the usual dose. This meta-analysis is larger and more impressive than either, but also less useful, since it doesn’t distinguish l-theanine from caffeine and other substances very well.

This study finds l-theanine to be safe and effective for sleep in its target population of boys with ADHD.

So it seems like there is some modest formal support for breakthrough anxiety, sleep, and coadministration with caffeine, but less/conflicting formal support for chronic anxiety.

Informally, a survey of 380 supplement users asking about 60 supplements found that l-theanine was rated the sixth most effective overall at relieving anxiety, and the most effective substance that was non-addictive and easily available.

3. How does theanine work?

Theanine closely resembles glutamate, the most important activating neurotransmitter in the brain. To vastly oversimplify: theanine gets into most of the same places glutamate does, but remains inert, crowding out the real glutamate. Since glutamate is mostly an “activating neurotransmitter” (turns your brain on, makes it more alert), crowding it out of certain systems can calm you down.

For the much more complicated version, see this study for a discussion of exactly which glutamate-related receptors theanine affects and how, and this study for some effects on other systems.

4. Is l-theanine safe? What are the side effects of l-theanine?

It’s a truism that no substance has zero side effects. But l-theanine is about as close as you can get. Some sources say headache, but this seems rare. Others note that any anti-anxiety substance can make you a little slower and less sharp, although most people don’t notice this with theanine. You can probably assume that most likely you will not get any side effects from theanine.

People have been consuming low doses of l-theanine in tea for thousands of years without any noticeable problem. And studies have tested very high doses of 900 mg or more daily for several months and not noticed any problems there either. This is probably about as safe a chemical as you are likely to find.

–4.1. How does theanine in supplements compare to the theanine in tea?

The standard dose in supplements is 200 mg per pill. The dose in tea is usually less than 25 mg per teabag.

5. What brand of theanine is best? Is there a difference between theanine, l-theanine, and sun-theanine?

Many organic molecules come in two “mirror image” varieties, called “right-handed” and “left-handed”. Most research on theanine as a supplement focuses on the left-handed variety, also called l-theanine. The right-handed variety is probably inactive. For a while you could get either just “theanine” (a mix of both) or pure l-theanine (left-handed only), and the dose of l-theanine would be half that of theanine. Now almost everyone sells l-theanine, and people just use “theanine” as a shorthand way to talk about it.

Sun-theanine is l-theanine that’s made using a specific method. For a while it was the only method that could create pure l-theanine (as opposed to mixed left- and right- handed theanine), but now there are many other methods and sun-theanine isn’t any better than anything else. It’s just one brand with an especially long history.

L-theanine isn’t an especially complicated molecule and basically everyone gets it right. There is no major difference between brands, so buy whichever one is most convenient for you. If you can’t decide, this brand is inexpensive and as good as anything else.

6. How should I take l-theanine?

If you’re interested in something to make your coffee feel gentler and work better, take l-theanine with caffeine at between a 1:1 and 3:1 ratio. IE if you’re drinking a standard cup of standard coffee with about 100 mg caffeine in it, take between 100 and 300 mg of l-theanine with it. You can start with the lower dose and go up to the higher one if that doesn’t work. If you really want, you can buy products that combine caffeine and l-theanine and already have the right ratio, like these caffeine/l-theanine pills or one of the brands of coffee that already has l-theanine in it (though I refuse to link to any these because none of them tell you how much they have – if you know one that does, let me know and I’ll add it in).

If you’re interested in using it for breakthrough anxiety, take 200 mg when you’re feeling anxious and wait about a half hour for it to work. If 200 mg isn’t enough, next time you can take 400 mg. Most people would not take more than 400 mg a day, although studies imply there shouldn’t be any problem with taking 900 mg or more.

If you’re interested in using it for chronic anxiety, consider taking 200 mg in the morning. If that works but seems like not enough, you could potentially go up to 400 mg in the morning or 200 mg twice a day. Studies and anecdotes weakly suggest this might stop working after a while, ie you might develop tolerance. If this happens, you might want to switch to a five days on, two days off schedule, or even a two weeks on, two weeks off schedule.

If you’re interested in using it for sleep, take 200 – 400 mg just before bedtime.

Like many supplements, there seems to be a wide variety of levels of response to l-theanine. Some people say it changes their lives, other people say it has no effect for them. If you’ve tried it at a couple of different doses at various times and still don’t feel anything, move on.