L-Theanine 101: Usage, Health Benefits and Side Effects

What can L-Theanine do for your health?

L-theanine is a non-essential amino acid. Its richest source is tea (especially green tea), but it is also found in the bay bolete mushroom.

It is said to promote a range of health benefits, including better sleep, relaxation and attention. The compound is also thought to improve immune function.

It was discovered by Japanese scientists in 1949.

L-theanine is found in tea

Is it safe?

Although there is very little research on the long-term toxicity of L-theanine in humans, animal research suggests the compound is very safe.

Even at high doses (as much as 6500mg/kg), it failed to induce toxic effects[1].

In the US, the Food and Drug Administration (FDA) states that L-theanine is “generally recognized as safe (GRAS)[2].

Health Benefits

1) May improve mood and subjective well-being

According to research, L-theanine could reduce stress and make you feel more alert. It is thought to achieve this by raising the levels of dopamine, serotonin and gamma-Aminobutyric acid (GABA)[3].

The neurotransmitters are thought to help the brain better control emotions, improve mood and even reduce stress-induced comfort eating[4].

According to Nobre et al, L-theanine has been shown to relax ‘the mind without inducing drowsiness’[5].

One study by Williams et al noted the same effects, while suggesting the compound may also reduce resting heart rate[6].

Research by The Journal of Clinical Psychiatry also shows that L-theanine could help reduce anxiety in those suffering from schizophrenia[7].

In combination with caffeine, L-theanine is thought to increase alertness, which is a measure of the Bond–Lader VAS (mood-scale rating)[8].

2) Could enhance sleep quality

Experts also state that L-theanine could improve sleep in several ways.

First, by aiding relaxation, L-theanine is said to help you go to sleep faster. According to a 2019 study by Suhyeon Kim et al, the amino acid could help reduce sleep latency by 14.9%[9].

Second, during the same study subjects who took the compound showed a 26.8% increase in sleep duration.

Third (and perhaps most impressively), L-theanine was shown to increase rapid eye movement (REM) by 99.6% and non-REM (NREM) sleep by 20.6%.

In yet another study (from 2019), the amino acid was shown to reduce sleep disturbances in people with generalized anxiety disorder (GAD)[10].

3) May promote relaxation

L-theanine is said to decrease the resting heart rate, which is why both green tea and black tea are thought to contribute to your ability to relax[11].

Man relaxing

4) Said to increase attention

L-theanine has also been shown to boost attention and focus.

In isolation, the compound could help improve reaction times and raise ‘visual attentional performance’[12]. In its own right, L-theanine is thought to be particularly effective for those suffering with high anxiety.

But it is said to really shine when paired with caffeine. During one 2013 study, the mix of caffeine and L-theanine reduced tiredness, increased reaction times and improved attention during a ‘demanding cognitive task’[13].

Also known as ‘Smart Caffeine’, the L-theanine/caffeine combo may also reduce impulsivity and increase sustained attention in children with attention deficit hyperactivity disorder (ADHD)[14].

In another study (from 2012), the blend of caffeine (50mg) and L-theanine (100mg) appeared to promote fewer errors in a Sustained Attention to Response Task (SART) during four two-hour sessions on consecutive days[15].

5) Thought to sharpen memory

Although L-theanine is not widely studied for this purpose in humans, some animal research suggests L-theanine could protect against brain cell death, which in turn may help improve and preserve the memory[16].

In the study in question (by Egashira et al), a dose of 1mg/kg also prevented loss of spatial memory. 

In human research by Haskell et al, subjects who took a combination of L-theanine and caffeine reduced word recognition times, improved ‘sentence verification accuracy’ and increased working memory[17].

6) Could support the immune system

L-theanine could help support immune function in many ways. It has been shown to reduce upper respiratory infections (especially those caused by colds and flu)[18].

According to one study, the incidence of influenza infection was reduced by 4.1% in those who took L-theanine (compared to those who took a placebo) during a five-month study between 2009 and 2010[19].

The non-protein amino acid is also thought to lower inflammation in the intestines[20] and reduce the symptoms of allergic reactions[21].

The reason for L-theanine’s potential immune-boosting effects may come from its ability to fight bacteria[22].

An article in the New York Times states that blood cells of tea drinkers ‘responded five times faster to germs than did the blood cells of coffee drinkers’[23].

7) Investigated as a cancer treatment

While more conclusive research is required to verify this, L-theanine may also have the potential to help fight cancer.

In one study by Qian Liu et al, theanine was said to suppress ‘A549 cell invasion’, which could have broad therapeutic application in ‘the treatment of human lung cancer and leukemia’[24].

It is believed that L-theanine could be the reason that Asians have a lower rate of cancer and heart disease, despite a larger proportion of the population being smokers of cigarettes[25]. This is because, as previously stated, L-theanine is found in green tea – a popular drink in Asia.

In other research in China, women who had ovarian cancer and drank at least one cup of green tea per day lived longer than women who didn’t drink any[26].

In yet another study, men and women who drank green tea were found to be less likely to develop pancreatic cancer[27].

Despite a lack of human research, one animal study even suggests L-theanine could enhance the positive effects of doxorubicin – a chemotherapy drug[28].

8) May reduce blood pressure

L-theanine is also thought to lower blood pressure.

High blood pressure may increase the risk of heart attacks and strokes, as well as mild cognitive impairment[29] and dementia[30]. It could even cause problems with the kidneys[31], eyes[32] and sexual function[33].

L-theanine could promote relaxation, while also attenuating blood pressure increases in adults subject to high stress. Also thought to reduce the heart rate, the compound could help protect the user against many of other issues mentioned above.

9) Potential to promote weight loss

L-theanine could promote weight loss in three ways:

  • Suppressing the appetite – L-theanine is thought to contain umami, which may speed up the metabolism and boost a sense of fullness to reduce hunger cravings[34].
  • Reducing the desire to comfort eat – Possibly as a result of its supposed ability to promote relaxation, L-theanine could also reduce stress-related eating.
  • Cutting fat accumulation –  The amino acid is said to reduce serum concentrations of triglycerides and other fatty acids[35].

10) May be neuroprotective

L-theanine has a similar chemical structure to glutamate – an important excitatory neurotransmitter in the brain[36], which is associated with memory. As a result, it has been researched for its ability to reduce age-related mental and physical decline.

In fact, a study by Kakuda suggests that “long-term ingestion” of L-theanine may help prevent dementia in the elderly[37].

As further evidence of L-theanine’s possible neuroprotective effects, animal research by Zukhurova et al suggests it could prevent stroke or decrease the long-term effects of such an event[38].

11) Possible treatment for schizophrenia

In an eight-week study of people of both genders (aged 18-29 and 30-44) who were diagnosed with DSM-IV schizophrenia, L-theanine appeared to reduce activation symptoms and reduce anxiety.

What’s more, it seemed to do this without affecting other markers of general functioning or quality of life[39].

Side Effects

As we state earlier in this article, L-theanine is on the FDA’s list of ingredients ‘generally recognized as safe (GRAS)’.

Furthermore, dosages of up to 6500mg/kg failed to cause toxicity during a study of its effects[40]. As a result, it may be safe to take.

However, the amino acid could cause the following mild side effects:

  • Headache
  • Sleepiness

Improving your sleep may help your weight loss effort

As the main source of L-theanine is green tea, this may also cause side effects due, not to L-theanine, but to the presence of caffeine in the drink. These could include:

  • Irritability
  • Nausea
  • Digestive issues

Dosage considerations

According to drugs.com, studies on the anxiolytic effects of L-theanine tend to use single doses of between 200mg and 250mg[41].

Short-term studies on L-theanine’s effects on psychiatric disorders (including schizophrenia) have used servings of between 240mg and 400mg per day (in separate doses).

When used in combination with 30-100mg of caffeine, L-theanine tends to be taken in doses of 12-100mg[42].

Interaction, Risks and Precautions

According to webmd.com, L-theanine may have ‘moderate interactions’ with certain medications. These include the following:

Stimulants: Stimulant medications are often prescribed for attention deficit hyperactivity disorder (ADHD) and narcolepsy. Such drugs are said to work by speeding up the nervous system.

As L-theanine is thought to slow down the nervous system, it could reduce the effectiveness of these medications, which may include methylphenidate (sold as Ritalin and other brand names) and modafinil (widely sold as Provigil in the US).   

Antihypertensive drugs: Theanine is thought to reduce blood pressure. When combined with antihypertensive drugs, this could reduce blood pressure too much.

The respected website lists captopril, enalapril and furosemide among the medications which could interact with the amino acid in this way.


As with any supplement, it’s important to ask questions before you take it. Here we answer some of the most common questions about L-theanine…

1. What is L-theanine?

L-theanine is a non-essential amino acid. It is found in tea (black and green) and bay bolete mushrooms. It is said to promote relaxation without sedation.

2. What should I take L-theanine with?

Due to its anxiolytic effects, L-theanine is thought to work synergistically with caffeine to enhance focus and attention, while reducing jittery effects. As a result, it may be useful for those who need an energy boost but are sensitive to stimulants.

3. Will L-theanine react with my medication?

L-theanine may have mild interactions with antihypertensive and stimulant medications. For more information, go to the Interaction, Risks and Precautions section of this article.

2. When should I take it?

There do not appear to be any strict rules as to when to take L-theanine. But according to an article on psycologytoday.com, the amino acid could begin to take effect within 30 minutes of ingestion[43].

For students wishing to increase mental performance, webmd.com also suggests it may work best when taken immediately before a test[44].

4. Can L-theanine cause anxiety?

The available research suggests this is highly unlikely. In fact, a number of studies have shown that L-theanine is a potential anxiolytic which could improve mood and enhance subjective well-being.

5. Is it safe?

L-theanine lacks enough long-term studies to say for sure whether it is safe. That said, there have been no reported L-theanine overdoses. In addition, extremely high doses of 6500mg/kg failed to cause toxicity in an animal study[45].  

The Final Word

A great deal of research suggests L-theanine is a very promising amino acid with a wide range of potential benefits.

Anything bad to say about it?

Perhaps the worst thing to say about L-theanine is that it is short of long-term research on its effects. It may also increase the effects of certain high blood pressure medications and reduce the effectiveness of stimulants (see reference 44).

However, the amino acid has also been found to both enhance the positive effects of caffeine (focus, attention) and reduce the negative issues that may otherwise arise from taking the stimulant (such as jitteriness and anxiety).

What’s good about L-theanine?

Research suggests L-theanine confers a wide range of potential benefits. Its possible ability to help you relax and improve sleep quality are perhaps the best-known and best-researched of its proposed uses.

However, it may also reduce age-related cognitive decline, prevent high blood pressure and promote weight loss.

Some preliminary research even suggests the amino acid may have some anti-cancer effects that could complement some relevant medications.

On this last point, it’s important to note that further studies are required to confirm L-theanine’s potential to help prevent cancer.

It’s clear L-theanine holds great promise as a supplement for a range of uses – and it’s perhaps no wonder it’s so popular in pre-workouts, weight-loss products and single-ingredient supplements.



[1] https://examine.com/supplements/theanine/

[2] https://pubchem.ncbi.nlm.nih.gov/compound/L-Theanine

[3] https://www.ncbi.nlm.nih.gov/pubmed/17182482

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6178259/

[5] https://www.ncbi.nlm.nih.gov/pubmed/18296328

[6] https://www.mdpi.com/2306-5710/2/2/13/htm

[7] https://www.researchgate.net/publication/259893901_L-Theanine_in_Schizophrenia_and_Related_Disorders_J_Clin_Psychiatry_e1_L-Theanine_Relieves_Positive_Activation_and_Anxiety_Symptoms_in_Patients_With_Schizophrenia_and_Schizoaffective_Disorder_An_8-Wee

[8] https://academic.oup.com/ajcn/article/98/6/1700S/4577506

[9] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6366437/

[10] https://www.ncbi.nlm.nih.gov/pubmed/30580081

[11] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3518171/

[12] https://www.sciencedirect.com/science/article/abs/pii/S1756464611000351

[13] https://www.tandfonline.com/doi/abs/10.1179/147683010X12611460764840

[14] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6574559/

[15] https://www.ncbi.nlm.nih.gov/pubmed/22326943

[16] https://www.ncbi.nlm.nih.gov/pubmed/17705146

[17] https://www.ncbi.nlm.nih.gov/pubmed/15678363

[18] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3275984/

[19] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3049752/

[20] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5622680/

[21] https://www.ncbi.nlm.nih.gov/pubmed/27908701

[22] https://academic.oup.com/ajhp/article/63/1/26/5134445

[23] https://www.nytimes.com/2003/04/22/us/study-concludes-tea-helps-fight-infection.html

[24] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2774573/

[25] https://cebp.aacrjournals.org/content/15/12_Supplement/A157

[26] https://pdfs.semanticscholar.org/3cd8/5853b8ec413dad582049a371fc4f652d0175.pdf

[27] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3490023/

[28] https://www.ncbi.nlm.nih.gov/pubmed/10037191

[29] https://www.nia.nih.gov/news/high-blood-pressure-linked-cognitive-decline

[30] http://www.bloodpressureuk.org/BloodPressureandyou/Yourbody/Dementia

[31] https://www.heart.org/en/health-topics/high-blood-pressure/health-threats-from-high-blood-pressure/how-high-blood-pressure-can-lead-to-kidney-damage-or-failure

[32] https://www.webmd.com/g00/hypertension-high-blood-pressure/guide/eye-disease-high-blood-pressure?i10c.ua=1&i10c.encReferrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS8%3d&i10c.dv=24

[33] https://www.mayoclinic.org/diseases-conditions/high-blood-pressure/in-depth/high-blood-pressure-and-sex/art-20044209

[34] https://www.psychologytoday.com/gb/blog/sleep-newzzz/201708/what-you-need-know-about-l-theanine

[35] https://www.ncbi.nlm.nih.gov/pubmed/15011752

[36] https://www.grc.com/health/research/Theanine/Neuroprotective%20effects%20of%20theanine%20and%20its%20preventive%20effects%20on%20cognitive%20dysfunction.pdf

[37] https://www.grc.com/health/research/Theanine/Neuroprotective%20effects%20of%20theanine%20and%20its%20preventive%20effects%20on%20cognitive%20dysfunction.pdf

[38] https://www.ncbi.nlm.nih.gov/pubmed/23097345

[39] https://www.researchgate.net/publication/259893901_L-Theanine_in_Schizophrenia_and_Related_Disorders_J_Clin_Psychiatry_e1_L-

[40] https://examine.com/supplements/theanine/

[41] https://www.drugs.com/npp/l-theanine.html

[42] https://www.webmd.com/vitamins/ai/ingredientmono-1053/theanine

[43] https://www.psychologytoday.com/gb/blog/integrative-mental-health-care/201710/l-theanine-generalized-anxiety

[44] https://www.webmd.com/vitamins/ai/ingredientmono-1053/theanine

[45] https://examine.com/supplements/theanine/


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