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My experience with L-theanine: dose, benefits, side effects

If you’re researching L-theanine, also known as theanine, you may already know that it’s an amino acid found in green and black tea, that boosts mood, and promotes a sense of calm without drowsiness. I will let Aaron get into the science of how L-theanine works to create these benefits later in this post, but before I do, I want to describe how I felt when I took L-theanine for the first time.

How L-theanine made me feel

In a word, great. L-theanine gives me a calm, creative focus. I feel unrestrained by mental clutter and notice an improvement in mood.

For example, when I first tried L-theanine, I was visiting San Diego. When I returned the rental car, the people at Hertz ignored me for a very long time, taking their sweet time processing my car. This would have irritated me under normal circumstances (many years in NYC gives you this ridiculous sense of time when it comes to the performance of services, you want it now!). However, rather than indulge in a moment of self imposed stress, I waited patiently. When the Hertz man finally came over, I asked how his day was going.

I went into duck mode, water rolled off my back.

Overall happier, calmer, and less stressed. I felt a noticeable increase in creative energy. I normally have lots of ideas, but the increased focus allowed me to digest and organize, rather than feeling overwhelmed by all the things I wanted to create/ things I had to do.

As I used theanine over the course of a week, I found that it was also a meditation aide.

Since that original positive experience, I’ve noticed a diminishing return with theanine. Yes, it is still a calming agent, but the results aren’t as pronounced. Probably the biggest change I notice now when I do take theanine is better sleep some nights. My deep sleep and REM sleep as measured by my Oura Ring seem to improve, even when I have taken theanine in the morning.

I still experiment from time to time, but why don’t I take theanine on a regular basis any longer?

Researching how we make serotonin made me realize how delicate the balance is for amino acid uptake into the brain. I don’t want to rely on a supplement like theanine (which is shown to readily cross the blood brain barrier) so as to avoid the “whack-a-mole” scenario where the suppression of one amino acid leads to too much of another. In short, I am concerned that long term use of theanine can throw off my brain chemistry in a way I won’t like.

L-theanine Health Benefits

There have been a number of studies looking at the benefits of L-theanine. The FDA considers theanine as a GRAS (“Generally recognized as safe”) supplement.

L-theanine is a Calming Agent

In the world of brain chemistry, glutamate is an “excitatory” neurotransmitter. You can think of glutamate as the gas pedal. By contrast, GABA is the calming agent in the brain.

Glutamate and GABA are both essential neurotransmitters, with important roles to play in cognitive function, however, when glutamate levels get too high, as they can during times of prolonged stress, when GAD1 mutations are present, or on a high glutamate diet, the usually beneficial glutamate can become an excitotoxin.1

In fact, elevated glutamate has been linked to a number of neurological diseases.2

This study found that L-theanine reduces physiological and psychological stress by blocking glutamate receptors in the brain. One of the primary reasons people experience an anti-anxiety effect when taking theanine is due to this glutamate blocking activity. Put simple, L-theanine helps take your foot off the gas pedal.

L-theanine May Improve Sleep

In a randomized, double blind study, theanine was shown to significantly improve sleep quality in boys with ADHD.3

L-Theanine Boosts Immune Function

This study found that L-theanine prevented the cold and flu and had an immune boosting effect “enhancing gamma-delta T cell function.”

The New York Times, reporting on the study, had this to say in summary:

Tea increases the body’s defenses against infection and contains a substance (L-theanine) that may be turned into a drug to protect against disease, a study has found. Coffee does not have the same effect, the researchers say in an article today in Proceedings of the National Academy of Sciences. A component in tea primes the immune system to attack invading bacteria, viruses and fungi, the study concluded. An experiment showed that immune system blood cells of tea drinkers responded five times faster to germs than did the blood cells of coffee drinkers.

Theanine increases Alpha Brain Waves

Alpha brain waves are the creative brain waves that we could all stand to benefit from a little more of. Studies show L-theanine improves alpha brain wave function, even at low 50 mg doses meant to mimic “normal dietary levels.”4

L-theanine and allergy

L-theanine has shown some promise as a modulator of allergic reactions.5

A major event in allergic reactions is the secretion of histamine from immune cells known as mast cells. Mast cells can be thought of as the first responders of the immune system, they are constantly sensing the environment and when they detect something potentially harmful they secrete a wide range of factors to kick start the immune response. When mast cells detect something as harmful (even if it’s harmless) this causes an allergic reaction.6 Theanine inhibits histamine release from mast cells.

Interestingly, this inhibition of histamine also ties in with L-theanine’s glutamate inhibitory activities. Histamine can induce glutamate release, which as we’ve established above, is an excitatory neurotransmitter our bodies need.7 Excess histamine may cause an unhealthy build-up of glutamate, although there is some evidence that there is an upper maximum of glutamate build-up associated with histamine.

L-theanine and hangovers

I have found that theanine can also be a fun supplement to take socially. I will normally take 100mg prior to a night of drinking as it can reduce the effect of a hangover, and adds to the relaxing effects of alcohol.

How does theanine reduce hangovers? It appears theanine speeds the metabolism of alcohol by helping the body produce glutathione, although the study we cite to here is in mice and not yet established. However, I can attest anecdotally to theanine being helpful at reducing the impact after a night of drinks.

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L-theanine Side Effects

Low Blood Pressure

Theanine produced solid benefits for me, both in work and socially, and I will continue using it every so often, however, it is contraindicated for those on high blood pressure medication because it lowers blood pressure.8 When I’ve taken high doses for a number of days in a row, I have noticed a few episodes of light headedness after standing.

May Interfere With Chemotherapy Drugs

Memorial Sloan Kettering Hospital has sounded the alarm that theanine may reduce the effectiveness of some chemotherapy drugs.

Upset Stomach

There are many anecdotal reports of an overdose of theanine causing upset stomach in people experimenting with the supplement.

How L-theanine works

So the interesting questions then becomes how does L-theanine link all these together, and is there a single gene or SNP which is of particular interest?

Lets look at histamine first, a major gene involved in histamine metabolism is AOC1, which encodes for the enzyme Di-amine Oxidase (DAO – pay attention to that abbreviation as it’s the cause of much confusion). In health DAO functions to break down histamine following its release from mast cells, curtailing the immune response. There are several SNPs within AOC1 which are associated with reduced DAO activity, resulting in histamine intolerance, which is characterized by symptoms often associated with allergy such as headaches, flushing of the skin and irritation.

The benefit of L-theanine here is clear. By suppressing histamine release from mast cells, it is prevented from building up to harmful levels, even in those with reduced DAO activity.9

We’ve already described how L-theanine can inhibit stress by blocking glutamate receptors. As excess histamine can lead to the buildup of glutamate there’s a clear mechanism for impaired DAO activity leading to increased histamine, leading to increased glutamate. L-theanine works by both preventing histamine release, and also blocking glutamate activity as well.


There’s a nice indirect mechanism linking DAO, histamine and glutamate.

However, if you read around on the internet, you may find articles discussing a direct action for DAO interacting with glutamate. While there may be evidence linking them directly I’ve not come across it; rather I think it’s a case of confusing DAO the enzyme with DAO the gene. DAO is encoded for by the AOC1 gene and functions to breakdown histamine. There is however a DAO gene which confusingly encodes for an enzyme called  d-amino acid oxidase (DAAO), which is involved in glutamate metabolism.10

See also: You say DAO, I say DAAO

Quite why it seemed a good idea for DAO to be encoded for by AOC1 and DAAO to be encoded for by DAO I’ll never know, but I think this is the basis for much confusion. So be careful when you’re trying to interpret your genetic results!

L-theanine Dosage

I have found L-theanine to be an effective nootropic that produces a calm focus. For me, it’s an effective tool for writing, meditating, creative bursts, and even to unwind and get to sleep at the end of the day. I also like it socially.

If you’re using theanine as an anti-anxiety supplement, the calming effects for me were greatest when paired with GABA and valerian. Keep in mind that both GABA and valerian can upset the stomach, so start slow. I also don’t recommend this stack for daily use over extended periods of time, best to cycle on and off.

As we discussed in the side effects section, theanine lowers blood pressure. Because there can be side effects, I would encourage everyone to experiment with different doses to see what amount of theanine suits them best. Some will tolerate 200mg quite well, while others may want to dose more in line with the studies that used 50mg.

Key Takeaways

L-theanine is a potent nootropic that is widely regarded as safe. Many benefits have been shown from supplementing with theanine with stress reduction leading most people’s lists.

Do keep in mind that theanine can interact with some drugs and may lower blood pressure to a level that will be uncomfortable for some.

As theanine crosses the blood brain barrier, also be on the lookout for changes in mood after high doses.

John O'Connor

John O'Connor is the founder of Gene Food, host of the Gene Food Podcast and a health coach trained at Duke's Integrative Medicine Program. Read his full bio here.

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