- It has been reported that taking L-theanine may affect indicators such as the body's response to stress and sleep quality.
- The evidence score is 68, and reliability is moderate.
- This is research data aimed at people who experience everyday stress or who are concerned about their sleep quality.
L-theanine is a specific substance.
It is generally treated as something that is taken as a supplement/dietary component.
It has been suggested that taking L-theanine may change how the body responds and how one feels when experiencing stress.
For example, there are reports that when a situation is deliberately set up to induce stress in an experiment, changes are seen in heart rate and in certain components contained in saliva. In this study, which involved a relatively small number of 12 participants, the tendency was shown for the body's stress response to be suppressed compared with taking a placebo (a fake substance that looks like a drug but contains no active ingredient). In other words, a possible link to physical responses at the moment of sudden stress is being considered.
Effects in mentally busy situations, such as handling multiple tasks at once, have also been examined. In this study, involving 34 healthy adults, subjective stress felt one hour after intake was found to be lower than after taking the placebo substance. This study was conducted using a randomized controlled trial (a reliable research method in which participants are randomly divided into groups to compare effects). In other words, a possible effect on short-term load is also being examined.
Research has also been conducted on the relationship with sleep quality and mood, matters people are often concerned about in daily life.
In a study of 30 healthy adults who continuously took 200mg daily for 28 days, changes were seen in an indicator measuring sleep quality. In addition, there are reports of a decrease in scores on scales that assess the degree of anxiety and low mood. These results were shown with statistical significance (a value indicating that the observed result is likely to be a meaningful difference rather than due to chance). In other words, the possibility that continued intake affects sensations such as sleep is being examined.
Experiments using mice have also studied effects at a more microscopic, cellular level of mechanism.
In an experiment using model mice related to heart tissue, an effect of suppressing cell damage was suggested. This research relates to apoptosis (a mechanism by which cells die through a specific process) and oxidative stress (damage to cells caused by reactive oxygen species and similar substances generated in the body). In other words, it has been reported that, by suppressing the damage cells receive, tissue damage may be reduced.
| Study | Design | Number of participants | Duration | Main result |
|---|---|---|---|---|
| Biological Psychology (2007, PMID:16930802) | Randomized controlled trial (RCT) | 12 people | Unknown | A positive effect was reported |
| Nutrients (2019, PMID:31623400) | Randomized controlled trial (RCT) | 30 people | 28 days | A positive effect was reported |
| Nutrients (2016, PMID:26797633) | Randomized controlled trial (RCT) | 34 people | Unknown | A positive effect was reported |
| Molecular Medicine (Cambridge, Mass.) (2024, PMID:38943069) | Animal study | Unknown | Unknown | A positive effect was reported |
The study subjects include 12 adults in an experimental setting, 30 healthy adults with no mental illness, and 34 healthy adults aged 18 to 40. Research has also been conducted using MIRI (myocardial ischemia-reperfusion injury) model mice.
These studies are mainly conducted on healthy adults. Therefore, they can serve as a reference for healthy adults who feel stress in everyday life when considering their own situation.
This data includes 4 studies. Of these, 3 are randomized controlled trials (RCTs) and 1 is an animal study.
Regarding subjects, in addition to human adults, a study using mice is included. Therefore, not all results can be directly applied to humans.
The study conditions also varied widely, from those conducted over a specific period or dose to those conducted under an experimental setting.
What effect on stress has L-theanine been reported to have?
Studies have been reported showing that taking L-theanine reduces psychological and physiological stress responses. Specifically, this includes effects on heart rate and on the response of secretory immunoglobulin A in saliva.
What is known about the relationship between L-theanine intake and sleep?
In a study of healthy adults with no mental illness, it has been shown that scores on an indicator of sleep quality decreased after taking L-theanine. This is reported in connection with 4 weeks of intake.
What types of research exist on L-theanine?
Both randomized controlled trials (RCTs) in humans and animal experiments using mice have been confirmed. The evidence score is 68.
The safety of L-theanine has been examined in several clinical trials involving human subjects.
No serious adverse events have been reported. An analysis published in 2026 that pooled 31 randomized controlled trials (a total of 1,168 participants) compared adverse events against a group that took a placebo (a substance that looks the same but contains no active ingredient), and concluded that "no serious adverse events were reported." In individual trials, some mild issues such as minor gastrointestinal discomfort, headache, and irritability were reported, but their frequency was reported not to differ greatly from the group that took the placebo. That said, it may not suit everyone, depending on individual constitution.
Intake amounts examined in research. The amounts used in trials were mainly 200-400mg per day, and a trial using up to 900mg over 8 weeks has also been reported. On the other hand, this review did not find an upper daily intake limit set by any public authority. This does not mean "there is no limit" — it refers to the fact that no limit has been established.
For those taking medication. Regarding interactions with antihypertensive drugs (drugs that lower blood pressure), stimulants, sleep medication, and similar, there are theoretical concerns raised, but this review did not find clinical research or reports that actually confirmed these. As a precaution, those currently taking such medications should consult a doctor or pharmacist before taking L-theanine.
For those who are pregnant or breastfeeding, and for children. Regarding safety for those who are pregnant or breastfeeding, and for children, sufficient human trial data was not found, so it has been confirmed as neither "safe" nor "unsafe." In fact, overseas application documents also exclude those under 18, and those who are pregnant or breastfeeding, from their scope. Anyone in these categories should not take it based on self-judgment, and should consult a doctor.
Regarding the regulatory situation, in the United States there are multiple GRAS notifications (a system in which the authorities respond with "no objection" to a company's own safety determination — this differs from safety approval by the authorities). In the European Union (EU), the approval procedure as a novel food had not been completed as of the time of this review.
All of the above is a summary based on the research and public materials that could be confirmed at this time. If you have concerns about your health, or have a pre-existing condition or are taking medication, please consult a doctor or pharmacist.
Research suggests that L-theanine is related to the stress response and may also affect indicators such as sleep quality. The evidence score to date is 68, and reliability is moderate. However, this content is based on 4 studies, and since an experiment using mice is included, it does not apply to everyone. Why not start by reflecting on your own state with the self-check assessment?