L-theanine: what is it, benefits, side effects, dosage

last updated: Dec 16, 2021

5 min read

You may have heard that drinking tea is good for your physical and mental health, but why is that? One possible reason is the natural chemicals found in certain types of tea that have potential health benefits. For instance, the amino acid L-theanine is found in black and green tea, and it may come with a whole host of benefits. 

Keep reading to learn more about L-theanine and its health benefits. 

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What is L-theanine?

Theanine is an amino acid said to help relieve anxiety, stress, reduce insomnia, and boost mental performance. This amino acid is primarily found in green tea, black tea, and some types of mushrooms. 

The leaves and buds from the Camellia sinensis plant are commonly used to make tea and contain natural chemicals like theanine that get released into hot water while steeping. 

Theanine is also available as a dietary supplement as a pill or tablet. Theanine is found in two different forms, L-theanine and D-theanine. L-theanine is the version that’s usually used in nutrition supplements. 

What does L-theanine do?

L-theanine is considered an amino acid analog of L-glutamate and L-glutamine, meaning L-theanine has a similar structure to these other two amino acids. 

Glutamate is an amino acid produced by the body, and it transmits nerve pulses. The similarities in structure allow L-theanine to have similar functions as glutamate. Theanine may also block some of glutamate’s functions by interacting with the glutamate receptors. 

It’s believed L-theanine may influence mood and thinking by increasing several neurotransmitters, although more research is needed to prove these effects in humans. A double-blind, controlled trial suggests that taking L-theanine may help increase (Dodd, 2015)

  • Dopamine, which influences mood, motivation, and motor control

  • Gamma-aminobutyric acid (GABA), which slows down the excitability of nerve cells

  • Serotonin, which affects mood, happiness, and well-being

Benefits of L-theanine

Here are some of the potential benefits of L-theanine:

May boost memory, focus, and cognitive function

L-theanine may help boost cognitive performance like memory and problem-solving. One small study found that taking a supplement with both L-theanine and caffeine helped improve the participants’ ability to switch between tasks. The supplement also increased the participants’ feelings of alertness (Giesbrecht, 2010). 

Another study published in 2021 found that taking an L-theanine supplement may help improve attention, working memory and provide neuroprotective effects (Baba, 2021). 

However, not all studies have found improvements in cognitive performance, so more research is needed to truly understand these effects. 

May provide anxiety and stress relief

Sitting down with a cup of tea has long been associated with calming down and reducing stress. A 2017 study showed that drinking a low-caffeine green tea helped lower the stress response in students (Unno, 2017). While there are other compounds in green tea, it’s possible L-theanine contributed to these results. 

A more specific review of research on L-theanine found that taking 200 to 400 milligrams (mg) per day of L-theanine may lower stress and anxiety in people experiencing stressful conditions (Williams, 2020).

L-theanine is believed to have this effect on anxiety because it influences the neurotransmitters that influence mood. 

May boost the immune system

L-theanine may also support the body’s ability to keep you healthy and prevent illnesses. Some research suggests L-theanine may help with inflammation and reduce the risk of catching a cold by altering the balance of immune cells (Juszkiewicz, 2019; Li, 2016). 

May have anti-tumor effects

L-theanine has antioxidant effects, meaning it helps protect cells from damage caused by free radicals. Because of this, it’s possible that taking L-theanine could boost the effects of some anti-tumor drugs, like chemotherapy. The early research suggests this supplement may support cancer treatment and prevent tumor growth, though of course more research is needed to confirm these effects (Mao, 2019).

May reduce blood pressure

One effect of the stress response is an increase in blood pressure. Research suggests that L-theanine supplements may help reduce the effects of stress on blood pressure. A 2012 study found the supplement lowered tension and anxiety scores in people who experience high stress (Yoto, 2012).

May improve sleep

Many people struggle to get enough quality sleep every night. Research suggests L-theanine may be able to help. A 2019 study found taking L-theanine daily may help improve sleep by decreasing (Hidese, 2019):

L-theanine and caffeine

Combining L-theanine intake with caffeine has mixed results in the research. A 2021 study found a potential synergistic effect between L-theanine and caffeine, meaning they may be more effective at boosting mood and attention when taken together (Baba, 2021). 

However, another study found different results. This study noted that taking L-theanine with caffeine helped relax blood vessels. One of the effects of caffeine is it causes the small muscles in blood vessels to contract, which narrows the blood vessels. Taking a combination of L-theanine and caffeine may help reduce the contraction and leave blood vessels open, allowing blood to flow more freely (Dodd, 2015). 

While this effect is beneficial, the study found that the combination of L-theanine and caffeine also lowered caffeine’s usual effects on increasing alertness and concentration (Dodd, 2015). 

Since most green and black teas contain both caffeine and L-theanine, tea consumption provides both of these compounds. 

More research is needed to understand if taking these compounds separately is more beneficial than combined. It’s unclear if some of the beneficial effects noted in some studies could be caused by other compounds found in tea, such as catechins (Baba, 2021).

L-theanine for sleep

L-theanine has often been considered a safe and natural sleep aid. 

Unlike other sleep aids, L-theanine can be used at any time of the day because it doesn’t cause drowsiness. Instead, it helps promote sleep by creating highly relaxing feelings which may help with falling asleep and higher sleep quality (Rao, 2015).

A 2019 study found taking an L-theanine supplement of 200 milligrams (mg) per day reduced the time it took to fall asleep, the number of sleep disturbances, and the use of sleep medications (Hidese, 2019).

Researchers also explored the effects of this supplement on attention deficit hyperactivity disorder (ADHD) since sleep problems are a common complaint for people with ADHD. The clinical trial found that taking 400 mg of L-theanine daily helped the participants sleep longer and increase the quality of their sleep (Lyon, 2011). 

L-theanine dosage

Researchers have tested different doses and combinations of L-theanine supplements. Since there is no conclusive research around L-theanine, researchers haven’t been able to identify a specific recommended daily amount. 

The recommended dose may vary depending on your reason for taking L-theanine. It’s best to talk with your healthcare provider before starting to take any new dietary supplements and follow their medical advice. 

L-theanine side effects

L-theanine is generally considered safe, whether taken as a supplement or in your favorite cup of tea. While there aren’t any reported side effects of L-theanine specifically, common dietary supplements may lead to side effects like:

  • Nausea and vomiting

  • Upset stomach and bloating

  • Diarrhea or constipation

  • Headache

Because of the potential health effects of L-theanine, it may interact with some types of medical treatments, including stimulants or blood pressure medications. Taking L-theanine with medications that lower your blood pressure could cause low blood pressure. If you’re taking any prescription or over-the-counter medications, let your healthcare provider know before you start L-theanine so they can make sure there aren’t any concerning interactions. 

There isn’t enough research to know if L-theanine is safe during pregnancy or breastfeeding. So, it’s generally recommended to avoid taking L-theanine supplements just to be safe. 

It’s important to note that dietary supplements aren’t strictly regulated by the Food and Drug Administration (FDA), which means it’s up to supplement companies to ensure all its ingredients are safe and effective. For that reason, you should be sure to only get L-theanine supplements from a reputable company. 


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

  • Baba, Y., Inagaki, S., Nakagawa, S., Kaneko, T., Kobayashi, M., & Takihara, T. (2021). Effects of l-theanine on cognitive function in middle-aged and older subjects: A randomized placebo-controlled study. Journal of Medicinal Food , 24 (4), 333–341. doi: 10.1089/jmf.2020.4803. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8080935/

  • Dodd, F. L., Kennedy, D. O., Riby, L. M., & Haskell-Ramsay, C. F. (2015). A double-blind, placebo-controlled study evaluating the effects of caffeine and L-theanine both alone and in combination on cerebral blood flow, cognition and mood. Psychopharmacology , 232 (14), 2563–2576. doi: 10.1007/s00213-015-3895-0. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4480845/

  • Giesbrecht, T., Rycroft, J. A., Rowson, M. J., & De Bruin, E. A. (2010). The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness. Nutritional Neuroscience , 13 (6), 283–290. doi: 10.1179/147683010X12611460764840. Retrieved from https://pubmed.ncbi.nlm.nih.gov/21040626/

  • Hidese, S., Ogawa, S., Ota, M., Ishida, I., Yasukawa, Z., Ozeki, M., & Kunugi, H. (2019). Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: A randomized controlled trial. Nutrients , 11 (10), 2362. doi: 10.3390/nu11102362. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6836118/

  • Juszkiewicz, A., Glapa, A., Basta, P., Petriczko, E., Żołnowski, K., Machaliński, B., et al. (2019). The effect of L-theanine supplementation on the immune system of athletes exposed to strenuous physical exercise. Journal of the International Society of Sports Nutrition , 16 (1), 7. doi: 10.1186/s12970-019-0274-y. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6377763/

  • Li, C., Tong, H., Yan, Q., Tang, S., Han, X., Xiao, W., & Tan, Z. (2016). L-Theanine improves immunity by altering TH2/TH1 cytokine balance, brain neurotransmitters, and expression of phospholipase C in rat hHearts. Medical Science Monitor: International Medical Journal Of Experimental And Clinical Research , 22 , 662–669. doi: 10.12659/msm.897077. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4772912/

  • Lyon, M. R., Kapoor, M. P., & Juneja, L. R. (2011). The effects of L-theanine (Suntheanine®) on objective sleep quality in boys with attention deficit hyperactivity disorder (ADHD): a randomized, double-blind, placebo-controlled clinical trial. Alternative Medicine Review: A Journal of Clinical Therapeutic , 16 (4), 348–354. Retrieved from https://pubmed.ncbi.nlm.nih.gov/22214254/

  • Mao, X., Xiao, X., Chen, D., Yu, B., & He, J. (2019). Tea and its components prevent cancer: a review of the redox-related mechanism. International Journal of Molecular Sciences , 20 (21), 5249. doi: 10.3390/ijms20215249. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6862630/

  • Rao, T. P., Ozeki, M., & Juneja, L. R. (2015). In Search of a Safe Natural Sleep Aid. Journal of the American College of Nutrition , 34 (5), 436–447. doi: 10.1080/07315724.2014.926153. Retrieved from https://pubmed.ncbi.nlm.nih.gov/25759004/

  • Unno, K., Yamada, H., Iguchi, K., Ishida, H., Iwao, Y., Morita, A., & Nakamura, Y. (2017). Anti-stress effect of green tea with lowered caffeine on humans: a pilot study. Biological & Pharmaceutical Bulletin , 40 (6), 902–909. doi: 10.1248/bpb.b17-00141. Retrieved from https://pubmed.ncbi.nlm.nih.gov/28566632/

  • Williams, J. L., Everett, J. M., D'Cunha, N. M., Sergi, D., Georgousopoulou, E. N., et al. (2020). The effects of green tea amino acid L-theanine consumption on the ability to manage stress and anxiety levels: a systematic review. Plant Foods For Human Nutrition , 75 (1), 12–23. doi: 10.1007/s11130-019-00771-5. Retrieved from https://pubmed.ncbi.nlm.nih.gov/31758301/

  • Yoto, A., Motoki, M., Murao, S., & Yokogoshi, H. (2012). Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses. Journal Of Physiological Anthropology , 31 (1), 28. doi: 10.1186/1880-6805-31-28. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3518171/

How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

December 16, 2021

Written by

Ashley Braun, RD, MPH

Fact checked by

Steve Silvestro, MD

About the medical reviewer

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.