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8 L-Theanine Benefits + Dosage, Side Effects

Written by Carlos Tello, PhD (Molecular Biology) | Last updated:
Jonathan Ritter
Puya Yazdi
Medically reviewed by
Jonathan Ritter, PharmD, PhD (Pharmacology), Puya Yazdi, MD | Written by Carlos Tello, PhD (Molecular Biology) | Last updated:

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Tea

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

What Is L-Theanine?

L-theanine is an amino acid mostly found in tea leaves and fungi and used to help improve mood and lower stress, and to protect the brain and heart [1, 2].

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

Natural Sources of L-Theanine

How Does L-Theanine Work?

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

L-theanine increases brain serotonin and dopamine levels, which may improve memory and learning. It also increases the levels and effectiveness of GABA, the body’s main inhibitory neurotransmitter, and stimulates alpha brain wave activity, which may cause mental relaxation, concentration, and deep REM sleep [6, 7, 8, 9, 10, 11].

L-theanine may also increase glutathione levels, the body’s most important antioxidant molecule [12].

Given its similarity with the neurotransmitter glutamate, L-theanine may exert its neuroprotective effects by interfering with its function [6].

Benefits of L-Theanine

There are several L-theanine supplements commercially available, especially to curb anxiety, improve cognitive function, and boost immunity. However, the FDA hasn’t approved them for any conditions due to the lack of solid clinical research. Regulations set manufacturing standards for them but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing with L-theanine.

Possibly Effective for:

1) Brain Function

In a clinical trial on 30 healthy people, L-theanine improved verbal fluency and executive function. In 18 healthy people prone to anxiety, L-theanine improved attention [13, 14].

In a clinical trial on 46 people with minor brain damage, a combination of L-theanine and green tea extract improved memory during a mental exercise. The combination improved recognition memory, attention span, and alertness during a memory test [15].

L-theanine also improved recognition memory in rats, possibly by promoting brain development [16, 17].

A breakfast bar with L-theanine, caffeine, L-tyrosine, alpha-linolenic acid, vitamins and minerals improved alertness, attention, working and episodic memory, and executive function in a clinical trial on 95 people [18].

L-theanine protected the brain against age-related cognitive decline in 29 elderly people and against brain damage due to aging, stress, and Alzheimer’s disease in animal studies [19, 20, 21, 22, 23].

Combination with Caffeine

L-theanine and caffeine are typically found together in tea. L-theanine often enhances the cognitive improvement effects of caffeine while reducing its negative effects (such as increased blood pressure) [24, 25].

In 5 trials on 124 healthy people, this combination improved sustained and target-specific attention, attention switching, reaction time, numeric working memory, and alertness better than either substance alone [26, 27, 28, 24, 29].

This combination also reduced the damage caused by restored blood flow after a stroke (ischemia-reperfusion injury) in rats [30].

Taken together, there is some limited evidence to suggests that L-theanine may improve brain function, especially attention and memory. You may consult with your doctor if it may be helpful in your case.

2) Boosting Immunity

High cortisol caused by intense exercise shifts the Th1/Th2 balance towards Th2. This increases white blood cells (neutrophils) while decreasing immune cells (lymphocytes), which leads to inflammation and immunosuppression. L-theanine (both alone and combined with L-cystine) reverted these effects in 4 clinical trials on 66 athletes exposed to strenuous exercise [31, 32, 33, 34].

L-theanine, both alone and combined with L-cystine, strengthened the immune system and reduced the incidence of the flu and common cold in 3 clinical trials on 376 people. In another trial on 55 people, the combination improved the efficiency of the flu vaccination. Similarly, the combination increased antibiotic production and prevented the flu in mice [35, 36, 37, 38, 39].

L-theanine boosted the activity of immune T cells, which protect against infections and cancer, in cell-based studies. It also maintained a correct Th1/Th2 balance towards Th1, which enhances resistance to infectious microbes, in rats [40, 41, 42].

Although promising, the evidence is still limited to conclude for certain that L-theanine boosts the immune system. You may take it for this purpose if your doctor considers that it may help.

3) Anxiety and Stress

In 3 clinical trials on 56 healthy people challenged with stressful situations, L-theanine reduced anxiety symptoms, along with heart rates and blood pressure. Similarly, both a theanine-based nutrient drink and low-caffeine green tea reduced stress responses and salivary stress markers (cortisol and alpha-amylase levels) in 2 trials on 54 people [43, 44, 13, 45, 46].

However, a study on 16 healthy people found that L-theanine was only relaxing under resting conditions, but not during acute stress episodes. In line with this, this amino acid didn’t improve anxiety symptoms in 46 people with a diagnosis of generalized anxiety disorder [47, 48].

Chronic L-theanine, as an add-on to their current medication, reduced anxiety, sleep disorders, and cognitive impairment in a trial on 20 people with major depressive disorder [49].

In 2 clinical trials on 100 patients with schizophrenia, adding L-theanine to their antipsychotic medication reduced anxiety and other symptoms [50, 51].

In rats, L-theanine reduced anxiety, suppressed the stimulatory effects of caffeine, and enhanced the anti-anxiety effects of the sedative midazolam [52, 53, 54].

All in all, L-theanine may reduce anxiety in healthy people challenged with moderate stress or those diagnosed with major depressive disorder or schizophrenia according to the existing evidence. Discuss with your doctor before using L-theanine to improve anxiety and never use it instead of what your doctor recommends or prescribes.

4) Sleep Quality

Green tea with high L-theanine and reduced caffeine content improved sleep quality and reduced stress in 3 clinical trials on 50 middle-aged and 10 elderly people [55, 13, 56].

L-theanine may also help with sleep disturbances caused by mental conditions. It improved sleep percentage and efficiency in 2 clinical trials on almost 100 boys with ADHD, 20 people with major depressive disorder, and 17 people with schizophrenia [57, 49, 58].

In mice, a mixture of L-theanine and the neurotransmitter GABA increased sleep duration and quality [59].

In rats, L-theanine partly counteracted the sleep disturbances induced by caffeine [60].

Again, limited evidence suggests that L-theanine may help with sleep disturbances, especially in people with anxiety. You may discuss with your doctor if this amino acid may help increase your sleep quality.

Insufficient Evidence for:

1) Schizophrenia

As an add-on to conventional medication for schizophrenia, L-theanine improved positive, negative, activation, and anxiety symptoms of this condition in 3 clinical trials on almost 100 people [61, 58, 51].

Because all 3 clinical trials were small, the evidence is insufficient to support the use of L-theanine in people with schizophrenia. Larger, more robust clinical trials are required to confirm these results.

2) Preventing Heart Disease

L-theanine prevented blood pressure increase caused by stress or caffeine intake in 2 clinical trials on 64 healthy people [44, 25].

L-theanine decreased blood pressure in hypertensive rats and lowered blood fat levels in obese mice [62, 63, 64].

L-theanine may lower blood pressure by increasing nitric oxide production, as seen in cell-based studies. Because it also prevented LDL oxidation, it might help prevent artery narrowing [65, 66, 67].

Again, the existing evidence is insufficient to conclude for certain that L-theanine may help prevent heart disease. Further clinical research is needed.

3) Depression

In a clinical trial on 20 people with major depressive disorder, L-theanine as an add-on to conventional medication helped with depressive symptoms, anxiety, sleep disturbances, and cognitive impairments [49].

L-theanine also depression caused by chronic stress, stroke, in mice and rats [22, 68, 69, 70].

A small clinical trial and some animal research cannot be considered conclusive evidence that L-theanine helps with depression. More clinical trials on larger populations are required.

4) Adjunctive Cancer Treatment

The oral administration of L-theanine together with L-cystine reduced the adverse effects of adjuvant chemotherapy in a clinical trial on 70 people with cancer in the stomach and bowels (gastrointestinal cancer) [71].

L-theanine also enhanced the anti-tumor effects of chemotherapeutic medication while protecting healthy tissues from these drugs in animal and cell-based studies [72, 73, 74, 75].

All in all, the evidence is insufficient to determine if L-theanine has any value in adjunctive anticancer therapy. Further clinical research is needed.

Animal and Cell Research (Lack of Evidence)

No clinical evidence supports the use of L-theanine for any of the conditions listed in this section. Below is a summary of the existing animal and cell-based research, which should guide further investigational efforts. However, the studies listed should not be interpreted as supportive of any health benefit.

Antioxidant

The L-theanine in tea has dose-dependent antioxidant effects [76].

In multiple animal and cell-based studies, L-theanine protected the following organs from oxidative damage:

Weight Loss

In mice, L-theanine, along with other components of green tea, reduced triglyceride and fatty acid levels in the blood, as well as food intake and body weight [86].

In a study in rats, L-theanine reduced the uptake of sugars and fats in the gut [87].

Stomach Ulcers

L-theanine healed stomach ulcers caused by an anti-inflammatory drug (indomethacin) in mice at a dose of 10 mg/kg body weight. It reduced oxidative damage and balanced the immune response. However, L-theanine aggravated ulcers at a 4x higher dose [88].

Supplementing with L-Theanine

Dosage

Because L-theanine is not approved by the FDA for any conditions, there is no official dose. Users and supplement manufacturers have established unofficial doses based on trial and error. Remember to consult with your doctor if L-theanine might be helpful in your case and what dose you should take.

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, depending on their weight, is normally recommended.

Side Effects

This list does not cover all possible side effects. Contact your doctor or pharmacist if you notice any other side effects.

Call your doctor for medical advice about side effects. In the US, you may report side effects to the FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada, you may report side effects to Health Canada at 1-866-234-2345.

L-theanine was generally well tolerated in clinical trials and very few people experienced mild adverse effects such as headache, digestive issues, or sleep disturbances. Additionally, the frequency of these effects was similar in the treatment and placebo groups [35, 36, 48].

L-theanine can lower blood pressure. If you have low blood pressure or take medication with this effect, it can cause your blood pressure to drop too low.

Due to the lack of safety data in these conditions, pregnant and breast-feeding women should avoid L-theanine.

Always consult with your doctor before supplementing and let them know about any drugs and supplements that you are taking or considering to avoid interactions.

About the Author

Carlos Tello

Carlos Tello

PhD (Molecular Biology)
Carlos received his PhD and MS from the Universidad de Sevilla.
Carlos spent 9 years in the laboratory investigating mineral transport in plants. He then started working as a freelancer, mainly in science writing, editing, and consulting. Carlos is passionate about learning the mechanisms behind biological processes and communicating science to both academic and non-academic audiences. He strongly believes that scientific literacy is crucial to maintain a healthy lifestyle and avoid falling for scams.

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