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12 L-Theanine Benefits + Dosage, Side Effects

Written by Maja Stanojevic, MD | Reviewed by Nattha Wannissorn, PhD (Molecular Genetics) | Last updated:
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology) | Written by Maja Stanojevic, MD | Reviewed by Nattha Wannissorn, PhD (Molecular Genetics) | Last updated:

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L-Theanine is an amino acid commonly found in tea. Theanine, together with caffeine, produce a distinct cognitive enhancement profile without the jitters or other side effects. In addition, theanine has many other surprising health benefits. Read this post to learn more about the health benefits of theanine.

What is L-Theanine?

L-Theanine is an amino acid that is mostly found in tea leaves and fungi and is used to help improve mood and lower stress, and to protect the brain and heart [12].

Theanine usually refers to l-theanine, which has more health benefits compared to d-theanine.

Natural Sources of L-Theanine

  • Tea leaves [2, 3]
  • Japanese Camellia (green tea) [3]
  • Bay Bolete mushroom [4]

How Does L-Theanine Work?

L-Theanine is able to cross the blood-brain barrier and affect the brain directly. In the brain, it targets brain chemicals that regulate arousal, anxiety, pleasure, and attention [5].

L-Theanine increases brain serotonin and dopamine levels, with possible improvement in memory and learningIt also increases the levels as well as the effectiveness of GABA, the body’s main inhibitory neurotransmitter, and increases alpha brain wave activity, which causes mental relaxation, concentration, and deep REM sleep [6, 789, 10, 11].

L-Theanine also increases glutathione levels, the body’s most important antioxidant molecule [12].

L-Theanine and caffeine in combination are beneficial for improving performance on cognitively demanding tasks [13].

L-Theanine exerts neuroprotective effects possibly through inactivation of group 1 metabotropic glutamate receptors (mGluRs) [6].

Benefits of L-Theanine

1) Reduces Anxiety and Stress

After participants took l-theanine, they performed mental tasks. Compared to the patients who took a placebo, they had lower stress symptoms, along with reduced heart rates and blood pressure [14, 15].

Daily supplementation in schizophrenic patients leads to reduced anxiety and promoted relaxation in healthy patients [169, 3].

Supplemental doses of l-theanine that promote relaxation do not cause sedation [3].

In addition, l-theanine may also have stimulating effects when given at smaller doses, doses that are less than half the doses that promote relaxation [17].

2) Improves Brain Function

L-Theanine supplementation helps increase attention and improve memory.

In men and women who suffered from minor brain damage, a combination of l-theanine and green tea extract improved memory during a mental exercise. It did so by increasing delayed recognition during a memory test. The subjects’ attention span and alertness increased during the memory test as well [18, 18].

Improvement in attention has also been noted in healthy people who are prone to anxiety [19].

3) Reduces Negative Effects of Caffeine and Improves Its Cognitive-Enhancing Effects

L-Theanine and caffeine are typically found together in tea. A combination of caffeine and l-theanine together significantly improves alertness, reduce fatigue, and improves task-switching accuracy [20].

High dose caffeine (250 mg, equivalent to 2.5 cups of coffee) has some negative effects, such as jitters and increased blood pressure. L-Theanine was able to mitigate the increase in blood pressure [21].

L-Theanine decreases the negative and jittery effects of caffeine in rats. However, human studies found that l-theanine helps reduce caffeine jitters, although the changes were not significant [8, 22, 23].

L-Theanine changes the pharmacological profile of caffeine in humans, as the combination of caffeine and l-theanine have somewhat different cognitive enhancement effects [22].

4) Protects the Heart and Blood Vessels

L-Theanine improves blood vessel function by increasing nitric oxide production and also prevents fat peroxidation of LDL (low-density lipoprotein), which reduces the risks of hardening of the arteries [24, 2526].

L-Theanine also decreased blood pressure in hypertensive rats [27, 28].

5) Is an Antioxidant

The L-theanine in tea exerts a dose-dependent antioxidant effect [29].

It also restored the antioxidant capability of liver cells, which helps prevent liver injuries [30].

In rats, it dose-dependently decreased oxidative damage and stopped inflammation [31].

6) Increases Sleep Quality

ADHD may cause sleep disturbances. Boys who took two 200 mg doses of l-theanine daily for six weeks had a higher quality of sleep compared to ones who took a placebo [32].

They had higher sleep percentages and better sleep efficiency. Supplementation also decreased the amount of time the participants were awake during the night after sleep started [32].

7) Reduces Symptoms of Schizophrenia

Taking 400 mg daily for eight weeks resulted in less schizophrenic symptoms. The patients reported lower anxiety, less sensory complaints, and fewer muscle problems [16].

8) May Enhance Th1 Immunity and Combat Stress-Related Weakened Immunity

L-Theanine may boost the activity of T cells that protect against infection and tumors. It also shifts the Th1/Th2 balance towards Th1 [33, 3435].

The study showed that tea drinkers, when compared to coffee drinkers, had increased production of cytokines by T cells [33].

High cortisol generally suppresses the Th1 immune system and stimulates the Th2 immune system. In the presence of stress, l-theanine helps to reverse this trend. L-Theanine decreased suppression of the immune system and inflammation caused by high cortisol from endurance exercise [36, 37].

L-Theanine may help with allergies. It decreased the production of IgE and histamine release from mast cells [38, 39].

Studies showed that theanine intake prevents the flu and common cold [40, 41, 42].

L-Theanine intake also alleviates inflammation and promotes recovery after surgery [43, 44].

9) May Work As an Adjunctive Cancer Treatment

L-Theanine enhanced anti-tumor effects of cancer-killing drugs [45, 46].

It also reduced the toxicity of these drugs to healthy tissues [45, 46, 47].

L-Theanine decreased growth and migration of liver and lung cancer cells [48, 49].

10) May Reduce Symptoms of Depression

In an open-label study, l-theanine intake for eight weeks helped with depressive symptoms, anxiety, sleep disturbance, and cognitive impairments in patients with major depressive disorder [50].

11) Helps with Weight Loss

L-Theanine, along with other components of green tea, reduced triglyceride and fatty acid levels in the blood. The food intake and body weight decreased in the mice given theanine [51].

12) May Heal Stomach Ulcers

L-Theanine healed stomach ulcers at a dose of 10 mg/kg body weight, but aggravated ulcers at a higher dose of 40 mg/kg body weight [52].

It alleviated the adverse effects of non-steroidal anti-inflammatory drugs as an antioxidant and by stimulating prostaglandin E production [52].

Supplementing with L-Theanine


The appropriate dosage depends on the user’s age and health.

For an average weight adult, l-theanine is usually available in pills of 50 – 200 mg. Some people take up to 600 mg a day.

For children, a smaller dose such as 25 – 50 mg is good, depending on their weight.

Side Effects

In animal studies, oral intake of l-theanine did not produce toxic effects even at very high doses [53].

However, when taking theanine supplements, you should pay attention to your use of other drugs.

Theanine can lower blood pressure. If you take medication for blood pressure, your blood pressure might go too low.


Want More Targeted Ways to Enhance Brain Function?

If you’re interested in improving your cognitive function, we recommend checking out SelfDecode’s Limitless Mind DNA Protocol. It gives genetic-based diet, lifestyle and supplement tips that can help improve your cognitive function. The recommendations are personalized based on your genes.

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