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L-Theanine is a non-proteinogenic amino acid found in nature almost exclusively in the leaves of the tea plant (Camellia sinensis) and, in small amounts, in certain species of fungi. It is not involved in the formation of proteins in the human body and accumulates in the plant in its free form, where it accounts for a substantial proportion of the free amino acids in tea leaves (Juneja et al., 1999; Vuong et al., 2011). The substance was first described in 1949 by Japanese researchers studying nitrogen-containing compounds responsible for the characteristic taste of green tea, and shortly thereafter it was isolated from the leaves of the shade-grown tea “Gyokuro,” one of the most highly valued Japanese green teas and particularly rich in this amino acid (Juneja et al., 1999; Vuong et al., 2011). L-Theanine is one of the key substances that shape the taste experience of tea. On the tongue, similarly to glutamate, it activates the taste receptors responsible for “umami” and contributes to the full-bodied and slightly sweet taste of high-quality green teas (Juneja et al., 1999; Türközü and Şanlier, 2017). For centuries, traditional Asian tea culture therefore unknowingly made use of a substance whose calming yet non-sedating effects are now the subject of intensive research. An interesting paradox is that L-Theanine behaves similarly to glutamate on the tongue, while in the brain it appears to dampen glutamatergic neurotransmission (Kimura et al., 2007; Nathan et al., 2006). In the dietary supplement market, L-Theanine has the longest history of use in Japan, where it was approved as a food ingredient as early as 1964 without restrictions, except for use in infant foods, and where it is used in foods with FOSHU (Foods for Specified Health Uses) status for specific health-related applications (Juneja et al., 1999; Türközü and Şanlier, 2017). The best-known commercial form is the enantiomerically pure preparation “Suntheanine,” which is produced enzymatically and has been classified in the United States as GRAS (Generally Recognized As Safe) for use at up to 250 mg per serving (Borzelleca et al., 2006; Türközü and Şanlier, 2017). This regulatory history indicates that L-Theanine is a well-studied substance in terms of safety.
Chemically, L-Theanine is N-ethyl-L-glutamine, also known as γ-glutamylethylamide, with the molecular formula C7H14N2O3 and a molar mass of approximately 174.2 g/mol (Fig. 1). Its structure is closely related to glutamic acid and glutamine, which enables interactions with glutamatergic systems as well as transport across biological membranes via amino acid transporters (Türközü and Şanlier, 2017; Vuong et al., 2011). The dry matter of tea leaves contains approximately 1 to 2% L-Theanine, and the substance is readily extracted into hot water, meaning that a typical cup of tea provides approximately 8 to 30 mg depending on the type of tea, the amount used, and the brewing time (Juneja et al., 1999; Vuong et al., 2011). The cultivation method plays a key role. When tea bushes are shaded for several weeks before harvest, as is the case with “Matcha” and Gyokuro, the plant accumulates amino acids, including L-Theanine, in response to reduced photosynthesis while simultaneously decreasing its content of bitter catechins. This gives these teas their characteristic sweet taste and higher content of the active compound (Vuong et al., 2011). To reach the dosages used in clinical studies, a consumer would need to drink several cups of tea, which is why L-Theanine is also available in concentrated capsule form without caffeine. In the plant, L-Theanine is synthesized in the roots from glutamic acid and ethylamine and is subsequently transported to the leaves. Industrially, food-grade L-Theanine is now produced predominantly by enzymatic synthesis or microbial fermentation (Türközü and Şanlier, 2017; Vuong et al., 2011). Enantiomeric purity is crucial for quality. The natural L-enantiomer is biologically active and has been clinically studied, whereas cheaper synthetic preparations may contain a racemic mixture including D-Theanine, which is considered practically inactive (Vuong et al., 2011).
Figure 1.Tea plant (Camellia sinensis) and L-Theanine.
After oral administration, L-Theanine is rapidly absorbed in the small intestine. In a study involving healthy volunteers, its pharmacokinetics were comparable after taking a capsule and after drinking green tea. The maximum plasma concentration was reached after approximately 50 minutes, and the elimination half-life was about one hour, which corresponds with the practical observation that subjective and electrophysiological effects begin approximately 30 to 60 minutes after administration (Scheid et al., 2012). An important difference compared with some other calming substances is that, due to its structural similarity to neutral amino acids, L-Theanine can cross the blood-brain barrier via amino acid transporters and therefore act directly within the central nervous system (Nathan et al., 2006; Türközü and Şanlier, 2017), whereas the ability of orally administered γ-aminobutyric acid (GABA), for example, to reach the brain has long been debated (Boonstra et al., 2015). The proposed mechanisms of action are diverse and are partly based on preclinical models. Due to its similarity to glutamate, L-Theanine binds to glutamate receptors, where it acts as a weak antagonist and dampens excessive excitatory neurotransmission (Kimura et al., 2007; Nathan et al., 2006). Animal studies have also reported increased brain levels of GABA, dopamine, serotonin, and glycine, which have been associated with the relaxing effect and support of attention without feelings of nervousness. However, the exact mechanism behind these changes remains unclear, and their applicability to humans is uncertain (Nathan et al., 2006; Türközü and Şanlier, 2017). The most consistently reproduced finding in humans concerns its effect on electroencephalography (EEG). Administration of approximately 50 to 200 mg of L-Theanine increases activity in the alpha-wave range, which is associated with a state of relaxed alertness, meaning calmness without drowsiness and without negative effects on cognitive function (Juneja et al., 1999; Nobre et al., 2008). However, the relationship between changes in alpha activity and subjectively perceived calmness is not entirely clear, and in some studies these parameters did not correlate with one another (Nobre et al., 2008). In terms of stress reactivity, a classic study showed that a single dose of 200 mg reduced the physiological response to an acute psychological stressor. Specifically, it reduced heart rate and the salivary immunoglobulin A response, presumably by attenuating sympathetic activation (Kimura et al., 2007). Among multi-week interventions, a randomized placebo-controlled study involving 30 healthy adults is one of the most frequently cited. Daily administration of 200 mg for four weeks significantly reduced scores for depression, anxiety, and sleep disturbances as assessed using the Pittsburgh Sleep Quality Index (PSQI), while also improving verbal fluency and executive functions (Hidese et al., 2019). The cognitive effects of L-Theanine alone are moderate and appear to affect attention and reaction time more than memory. A meta-analysis of 31 randomized trials involving a total of 1,168 participants found that a single dose of 200 mg, administered 30 to 60 minutes before testing, significantly improved reaction time in attention-demanding tasks. L-Theanine was assessed as a safe substance with a robust short-term benefit for attention in healthy adults (Gerolymos et al., 2026). Another meta-analysis considers the evidence for cognitive effects promising, although not entirely conclusive (Mátyus et al., 2025). Consistent findings suggest that the combination of L-Theanine and caffeine is particularly interesting in practice. A number of randomized studies have repeatedly demonstrated improved accuracy when switching attention and increased subjective alertness, while L-Theanine reduces the nervousness and restlessness caused by caffeine (Giesbrecht et al., 2010; Haskell et al., 2008; Kelly et al., 2008; Owen et al., 2008). In practice, a ratio of approximately two parts Theanine to one part caffeine is therefore frequently recommended. Effects on sleep should be interpreted with caution, as L-Theanine is not sedating, and any potential improvement in sleep is thought to result from reduced evening restlessness and stress-related activation rather than suppression of the nervous system. The most recent systematic review of 13 studies involving a total of 550 participants concluded that supplementation with 200 to 450 mg daily appears to be a safe and effective way of supporting healthy sleep in adults, with beneficial effects on sleep onset, sleep continuity, and the feeling of restoration after waking (Cotter et al., 2026). However, it should be noted that most of the included studies were small and short-term and predominantly involved healthy individuals. A meta-analysis of tea interventions therefore evaluates the effects on sleep more cautiously (Cotter et al., 2026; Payne et al., 2025). Data in clinical populations are limited. In boys with attention-deficit/hyperactivity disorder (ADHD), 400 mg daily for six weeks improved some objective sleep parameters without affecting the core symptoms of the disorder (Lyon et al., 2011). In patients with schizophrenia, the addition of 400 mg daily to antipsychotic treatment for eight weeks reduced positive symptoms and anxiety (Ritsner et al., 2011), whereas in generalized anxiety disorder, adjunctive L-Theanine showed no superiority over placebo with regard to either anxiety or insomnia (Sarris et al., 2019). A systematic review of nine randomized trials in healthy individuals concluded that dosages of 200 to 400 mg daily may help reduce stress and anxiety in people exposed to stressful conditions, while also emphasizing the need for larger and longer-term studies (Williams et al., 2020).
Clinical studies have most commonly used dosages of 100 to 400 mg daily, with doses of up to 900 mg daily used in exceptional cases in certain populations (Cotter et al., 2026; Williams et al., 2020). A single dose of 200 mg approximately 30 to 60 minutes before a stressful situation or cognitive task is typical in studies examining attention and stress (Gerolymos et al., 2026; Kimura et al., 2007), while multi-week dosages of 200 to 450 mg daily have been used in studies focusing on sleep, mood, and psychiatric indications (Cotter et al., 2026; Hidese et al., 2019). L-Theanine has a favorable safety profile. A 13-week toxicity study in rats established the no observed adverse effect level (NOAEL) at 4,000 mg per kg of body weight per day. This was the highest dose tested, and the substance showed no genotoxicity (Borzelleca et al., 2006). Adverse effects reported in clinical studies have been mild and rare. Isolated cases of headache, dizziness, or mild digestive discomfort have been reported, including at dosages of up to 900 mg daily (Cotter et al., 2026; Hidese et al., 2019; Williams et al., 2020). For dietary supplement providers in the Czech Republic, the regulatory situation within the European Union is particularly relevant. The European Food Safety Authority (EFSA) evaluated proposed health claims for L-Theanine relating to cognitive function, psychological stress, sleep, and menstrual discomfort and rejected all of them due to an insufficiently established cause-and-effect relationship (EFSA, 2011). Consequently, no health claim for L-Theanine is authorized in the European Union, and product communication must be limited to describing the product’s composition without promising specific functional effects. The German Federal Institute for Risk Assessment (BfR) has also advised against adding isolated L-Theanine to beverages, as it considered the available data insufficient for a complete assessment of the isolated substance and highlighted the lack of data concerning children and pregnant women (BfR, 2003). Regarding potential interactions, L-Theanine reduces the stimulating effects of caffeine, which is generally considered desirable. Theoretically, however, it may slightly enhance the effects of antihypertensive medications and may have additive effects when combined with sedatives or anxiolytics (Türközü and Şanlier, 2017; Williams et al., 2020). There are insufficient safety data on isolated L-Theanine in pregnant and breastfeeding women and in children, and its use in these groups is therefore not recommended without medical consultation (BfR, 2003).
In practice, L-Theanine should be regarded as one of several options for managing stress and supporting sleep. A frequently compared substance is GABA, whose main limitation is the controversial ability to cross the blood-brain barrier after oral administration, meaning that the mechanism of any potential effects remains unclear (Boonstra et al., 2015). In this respect, L-Theanine has the advantage of having been shown to reach the brain (Nathan et al., 2006). Melatonin is the dietary supplement with the strongest evidence for reducing the time required to fall asleep. A meta-analysis of randomized trials reported a statistically significant, although modest in absolute terms, reduction in sleep latency as well as an improvement in sleep quality. It is particularly useful in disorders of the circadian rhythm (Ferracioli-Oda et al., 2013). Magnesium is involved in regulating neuromuscular excitability, and its combination with L-Theanine in the form of a complex has been associated with improved sleep parameters. However, the evidence for magnesium alone in insomnia is weaker and inconsistent (Dasdelen et al., 2022). Among adaptogens, Ashwagandha (Withania somnifera) has relatively solid supporting evidence. A randomized placebo-controlled study showed that a daily dose of 240 mg of standardized extract reduced anxiety and morning cortisol levels. One disadvantage is the variability of extracts available on the market (Lopresti et al., 2019). At the same time, it must be emphasized that no dietary supplement replaces treatment for chronic insomnia, as cognitive behavioral therapy for insomnia is considered the gold standard and recommended first-line treatment (Qaseem et al., 2016). L-Theanine and related supplements can therefore be considered supportive measures for mild, temporary, stress-related complaints, but not as treatments for clinical sleep disorders or anxiety disorders.
Written for Kratom World by:
Ing. Matěj Malík, Ph.D.
Researcher, Postdoctoral Fellow
Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences Prague
Department of Agroenvironmental Chemistry and Plant Nutrition
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