The tea on L-theanine: potential benefits, side effects and more
Written byEmily Coates
Reviewed byHeeral Tomlinson
Found in tea, L-theanine has been studied for its potential effects on relaxation, stress, focus and more. Here we break down the research, safety and how people are using it
Summary
1What’s L-theanine?
A naturally occurring amino acid found mainly in green and black tea. Pop the kettle on and let’s sip through the research.
2What are the potential benefits?
L-theanine has been studied for effects on relaxation, focus and sleep, though evidence is still developing.
3How to take L-theanine
You’ll find L-theanine naturally in teas like green tea, black tea and matcha. Sip it in your daily brew or try supplements for higher studied doses.
Ever wondered why your daily brew feels like a mini reset button? Beyond the comfort of a warm mug, some teas contain a lesser-known ingredient: an amino acid called L-theanine.
Researchers have long been interested in this naturally occurring compound. In recent years, it’s also started appearing in supplements, often marketed for areas such as focus, relaxation and sleep. But what does the science actually show? Pop the kettle on and let’s sip through the research.
What’s L-theanine?
L-theanine is a naturally occurring amino acid first identified in Japanese tea leaves in 1949.1 It is found most abundantly in green and black tea – especially matcha – and also appears in certain mushrooms. In other words, if you are a regular tea drinker, you may already be getting small amounts without even realising it.
Researchers have explored how L‑theanine interacts with the brain, including studies looking at its effects on neurotransmitters (brain chemicals that help your nerve cells communicate) and brain‑wave patterns. In simple terms, they're looking at how it could affect how your brain works behind the scenes.
What does L-theanine do?
Research has explored L-theanine in areas such as stress, sleep and cognitive function. Some studies have also examined whether it influences compounds like
serotonin and dopamine, or alters alpha-wave activity. However, findings are mixed2 – here’s what the evidence is brewing so far.
Focus and cognition
One of the most widely studied areas of L-theanine research is its potential role in attention and mental focus, particularly when paired with caffeine as it naturally is in tea. Several controlled studies have explored this combination, including trials assessing attention-switching tasks – where people switch their focus between different things, like answering emails while keeping track of a conversation – as well as alertness and subjective mood.3
Researchers have also examined how L-theanine and caffeine interact, focusing on whether the pairing influences stimulation and mental performance compared with caffeine alone.4 This combination, commonly consumed in tea, has been a key focus of research into cognitive function.
Other studies have looked at how the brain responds during thinking tasks after people have consumed L-theanine, caffeine, both or a placebo, using brain scans to track activity.5 Larger reviews have also brought together findings from multiple studies, exploring areas like attention, switching between tasks and working memory in relation to this combination.6
Overall, L-theanine has been widely studied in the context of cognitive performance, particularly alongside caffeine, but further high-quality research is needed to better understand its effects.
Stress and anxiety
Some studies suggest that L-theanine may help reduce stress responses, both psychological and physiological. For example, a controlled trial found participants who took L-theanine experienced lower stress markers and reduced heart rate responses during challenging tasks compared with placebo.7
Later reviews examining multiple studies have reported modest reductions in self-reported anxiety and stress, although the findings are not entirely consistent across all trials.8
Scientists suggest L-theanine’s potential calming effects may relate to its influence on brain chemicals and alpha-brain wave activity, though larger studies are still needed to confirm these effects in wider populations.9-10
Sleep
As L-theanine may promote relaxation, small studies have also explored whether it could support sleep. A 2024 systematic review and meta-analysis examining clinical trials found that L-theanine supplementation may improve certain measures of sleep quality.11
Importantly, L-theanine does not act like a sedative. Instead, it may help create a more relaxed mental state that makes it easier to wind down before sleep. As with many nutrition studies, more research is needed to determine the most effective doses and who may benefit most.
Is L-theanine safe?
For most healthy adults, L-theanine consumed through tea or supplements appears generally well tolerated. Reported side effects are uncommon but may include nausea, stomach upset or irritability at higher doses.12 Those who are pregnant, breastfeeding, have low blood pressure or are under 16 should avoid L-theanine supplements unless advised by a healthcare professional.
How to take L-theanine: food vs supplements
You can get your dose of L-theanine through certain teas such as green tea, oolong and black tea. These teas typically contain around 5–40mg of L-theanine per cup, although this varies depending on the type of tea and how it is brewed.13
Average amounts include:
- black tea: around 24mg per cup
- green tea: around 8mg per cup
- matcha: around 15–40mg per cup, depending on the quality and how much powder is used
By comparison, supplements usually provide around 200mg, which is the dose most often studied in research.
L-theanine FAQs
There are no official guidelines for daily L-theanine intake. Amounts can vary depending on the source, with tea typically providing around 5–40mg per cup, while supplements often contain around 200mg, a level commonly studied in research.
As evidence is still developing, it’s generally recommended to follow supplement labels and seek advice from a healthcare professional if needed.
Not exactly. L‑theanine is sometimes talked about in relation to sleep because researchers have explored whether it plays a role in relaxation or night‑time routines. Some people choose to use it in the evening based on personal experience, but its effects are not confirmed.
Ashwagandha is another popular ingredient. However, research on using both together is still limited, so it is best to speak to a healthcare professional first.
Interestingly, researchers have looked at whether L‑theanine might influence taste perception, with some studies examining its possible role in reducing bitterness in foods like chocolate or grapefruit.2
The final say
L-theanine continues to attract scientific interest, largely because it occurs naturally in tea and has been explored in research looking at attention, stress and sleep. While the findings are intriguing, the overall evidence is still developing.
If you’re curious to try it in its most natural form, tea is an easy starting point. Matcha, in particular, contains comparatively higher levels of L-theanine, which is why it’s often singled out in conversations around different tea varieties.
Want to explore further? Take a look at our guide to matcha powders or browse other well-known botanicals in our guide to ashwagandha supplements.
Disclaimer - This article provides informational advice and is not a substitute for medical care. Working with experts for accuracy, we take great care to ensure the information is up-to-date and relevant. However, you should always consult your GP or healthcare professional before using supplements or alternative products, particularly if you have medical conditions or are under supervision.
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