L-Theanine: An Overview of Effects, Dosage, and Side Effects

This article is for general informational purposes only. It does not replace professional medical advice, diagnosis or treatment. Always consult a doctor or pharmacist before taking dietary supplements. L-theanine is a dietary supplement — not a medicine.

What Is L-Theanine?

L-theanine is a non-proteinogenic amino acid found primarily in the leaves of the tea plant Camellia sinensis. It is present in green, black and white tea. In Japan and China, tea has been consumed for centuries as a beverage associated with calm and wellbeing.

Today, L-theanine is available as a dietary supplement in capsule form. As a supplement, it is not intended to treat, diagnose or cure any disease. It is best used as a complement to a healthy diet and active lifestyle.

What Does Science Say? — Research Overview

Studies on Stress and Relaxation

A double-blind, placebo-controlled study (2016) with 34 healthy adults investigated whether L-theanine could influence the stress response. Participants who took 200 mg of L-theanine showed reduced subjective stress perception, with cortisol levels measured at two hours post-intake.¹

A further study (2013) examined L-theanine in pharmacy students at the start of a placement. Salivary alpha-amylase activity — a physiological stress marker — was lower in L-theanine participants vs. placebo.²

Note: These studies identify possible associations; they do not prove medical efficacy in the pharmaceutical sense.

Studies on Brain Waves (Alpha Activity)

Several EEG studies have observed increased alpha brain wave activity following L-theanine intake. Alpha waves (8–12 Hz) are typically associated with a calm, relaxed waking state.⁷ This effect was particularly noted in individuals with higher baseline anxiety.

Studies on Sleep

A 2011 study in boys with ADHD administered 400 mg L-theanine daily and reported improved sleep efficiency scores.¹⁰ This study was conducted in a clinical patient population; applicability to healthy adults is limited. Dietary supplements should generally only be given to children following medical consultation.

Studies on L-Theanine + Caffeine

Multiple studies (2008, 2010, 2014) have examined the combined intake of L-theanine and caffeine. Results suggest possible improvements in reaction time, concentration and working memory.¹⁵˒¹⁶ Some studies indicate that L-theanine may attenuate caffeine-induced jitteriness and elevated heart rate.

Possible Effects of L-Theanine

Based on available research, the following areas are being investigated:

• Promoting a relaxed mental state without causing drowsiness
• Supporting stress management
• Contributing to healthy sleep quality
• Synergistic effects in combination with caffeine
• Possible support of cognitive alertness alongside caffeine

L-Theanine and Relaxation

The most consistently reported effect of L-theanine is promoting a relaxed state without causing sleepiness. In studies, this has been linked to increased alpha brain wave activity.

What Are Alpha Brain Waves?

The brain produces electrical oscillations of different frequencies measured in Hertz (Hz), categorised as:

• Delta (0.5–3 Hz): Deep sleep
• Theta (3–8 Hz): Light sleep, relaxed wakefulness
• Alpha (8–12 Hz): Calm, relaxed waking state — e.g. during meditation
• Beta (12–30 Hz): Focused attention, active thinking
• Gamma (25–100 Hz): Complex cognitive processing

Alpha waves represent the brain's default resting state. Increased alpha activity is associated with a calm, alert mindset — similar to the state after a brief meditation.

L-Theanine and Stress

Stress is a natural response to challenges. Chronic stress, however, can negatively impact wellbeing and daily life. Studies have investigated whether L-theanine may support stress management.

One study (2012) observed that L-theanine significantly reduced blood pressure rises in response to mental stress tasks compared to placebo.⁸ An earlier study (2007) reported lower physiological stress markers such as heart rate and immunoglobulin A after L-theanine intake.

These findings are promising but based on small studies. They indicate possible areas of interest — not proven therapeutic effects. If you suffer from significant or persistent stress, we recommend medical consultation.

L-Theanine and Sleep

Many people are interested in L-theanine as natural support for healthy sleep. Studies suggest that the relaxing properties of L-theanine might have a positive influence on sleep quality — without causing next-day drowsiness.

A review article (2019) concluded that L-theanine may contribute to relaxation and indirectly improve sleep quality, potentially as a result of its anti-stress effects.⁴

L-Theanine and Caffeine: Synergy

Tea naturally contains both L-theanine and caffeine — a combination that scientists have studied in depth. Caffeine is a stimulant that promotes alertness but can cause jitteriness and increased heart rate in some people.

Multiple studies indicate that L-theanine may attenuate these undesirable caffeine effects while preserving its alertness-promoting properties.¹⁵˒¹⁶ This combination is often chosen by people seeking a focused yet calm mental state — for example when studying or working.

Research-based combination guidance: 100–200 mg L-theanine with a cup of coffee or approximately 40–50 mg caffeine. Important: combinations with other substances or medications should be discussed with a doctor or pharmacist.

L-Theanine Dosage

General Dosage Reference from Studies

Most clinical studies have used doses of 100 to 200 mg of L-theanine per day. Some studies used 400 mg for specific research questions.

• Typical study dose: 100–200 mg, once or twice daily
• Starting dose: 50–100 mg — useful for assessing individual tolerance
• Higher doses (200–400 mg): used in some studies; please consult a professional

When to Take L-Theanine

• For general relaxation: at any time of day
• In combination with caffeine: morning or before focused work
• For sleep preparation: 30–60 minutes before bedtime

Side Effects and Safety

L-theanine is considered well-tolerated in studies at usual doses. Individual responses may vary. Known possible side effects include:

• Temporary headaches - particularly at higher doses or in combination with caffeine
• Mild gastrointestinal discomfort - especially on an empty stomach; taking after food may help
• Dizziness - rarely reported; pause intake if persistent
• Temporary slight lowering of blood pressure - relevant for those with already-low blood pressure

Possible Interactions

• Blood pressure-lowering medications - L-theanine may enhance hypotensive effects
• Stimulants (e.g. caffeine) - combination generally well tolerated; individual responses vary
• Sedatives / sleep medications - possible additive effects; medical advice recommended

General notice: If you take medications, are pregnant, breastfeeding or have a medical condition, please consult a doctor or pharmacist before use.

References

¹ Kimura K. et al. (2007): L-Theanine reduces psychological and physiological stress responses. Biol Psychol.

² Unno K. et al. (2013): Anti-stress effect of theanine on students during pharmacy practice. Pharmacol Biochem Behav.

⁴ Rao TP. et al. (2015): In search of a safe natural sleep aid. J Am Coll Nutr.

⁷ Nobre AC. et al. (2008): L-Theanine, a natural constituent in tea, and its effect on mental state. Asia Pac J Clin Nutr.

¹⁰ Anas Sohail A. et al. (2021): The Cognitive-Enhancing Outcomes of Caffeine and L-theanine. Cureus.

¹⁵ Haskell CF. et al. (2008): The effects of L-theanine, caffeine and their combination on cognition and mood. Biol Psychol.

¹⁶ Owen GN. et al. (2008): The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutr Neurosci.