The Impact of L-Theanine Supplements on Workout Performance: Benefits & Potential Side Effects

L-theanine is a powerful nootropic found in pre-workouts.

Supplements play a vital role in bridging nutritional gaps, supporting essential bodily functions, and enhancing overall health by providing key vitamins and minerals (1). Among them, pre-workout supplements stand out for their ability to boost exercise performance and effectiveness.

Modern pre-workouts have come a long way from simply relying on caffeine to energize the body. Many now include nootropics like L-theanine, which not only enhance physical performance but also sharpen mental focus. L-theanine works to reduce stress, improve cognitive function, and provide clean, sustained energy — helping to avoid the dreaded caffeine crash.

But do L-theanine supplements truly live up to the hype? Dive into this in-depth analysis to explore the benefits, potential drawbacks, and overall impact of this game-changing pre-workout ingredient.

Overview — L-Theanine Supplement

Theanine is a naturally occurring amino acid found in tea plants and mushrooms. These tea plants are the source of different tea types like ‌oolong tea, white tea, green tea, and black tea (2). Theanine consists of two parts: L-theanine and D-theanine, but L-theanine is the most common form.

It helps transmit nerve impulses in the brain. So many believe you can use the substance to improve your cognitive function, improve your sleep, and reduce stress and anxiety. That is why you can find the L-theanine component of it in supplement form, different tea variations, and mushrooms. It is essential not to confuse theanine with another amino acid called threonine.

L-Theanine Benefits

Improves Cognitive Function

When combined with coffee, it helps improve focus, retention, alertness, memory, and thinking skills. Researchers found that after young adults took caffeine and L-theanine, their focus, accuracy, and alertness improved (3).

Reduces Stress & Anxiety

It can help reduce stress and calm you down, making you more productive in high-stress situations. In this study of five randomized controlled trials, 104 subjects taking L-theanine resulted in a reduction of anxiety in stressful situations (4). Also, in another study involving people with schizoaffective disorder, taking L-theanine improved symptoms and reduced anxiety (5). 

Better Immunity

Taking L-theanine can help improve your bodily function by improving your immune system. Research shows subjects who took L-theanine supplements had a decrease in upper respiratory tract infections, flu, and enhanced inflammation in the intestinal tract (6) (7) (8).

Cancer & Tumor Treatments

Over time and with extensive studies, L-theanine has proven to help with increasing the anti-tumor effects of certain chemotherapy drugs. So many believe this supplement can enhance the chemotherapy’s ability to fight cancer (9). Regularly taking tea containing L-theanine can help lower and prevent cancer (10). However, more studies are needed in this area.    

Great Way to Regulate Blood Pressure

If you’re having high blood pressure, taking L-theanine can help reduce it, especially in stressful situations. This study found that in individuals who experienced high blood pressure after performing specific mental tasks, L-theanine reduced high blood pressure (11). 

Are There Any Side Effects?

In simple terms, L-theanine is generally safe to take with no significant side effects. However, as beneficial as L-theanine is to your optimal bodily functions and overall health, it is essential to be cautious of potential side effects. Regulation is key because some other properties can be harmful, especially to patients who are being treated for cancer.

For instance, in this study, green tea can block the anticancer effects of bortezomib and other boronic acid-based proteasome inhibitors (12). So, before taking green tea or L-theanine, it is essential to discuss with a certified health professional. 

Some other mild side effects are: slight headaches, stomach upset, irritations, and nausea. Women who are pregnant, breastfeeding, and have little children should try to limit their l-theanine supplement intake to avoid excess caffeine.   

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References

1. Should you take dietary supplements? (2024, June 17). NIH News in Health. https://newsinhealth.nih.gov/2013/08/should-you-take-dietary-supplements
2. Wang, Q., Yu, J., Lin, W., Ahammed, G. J., Wang, W., Ma, R., Shi, M., Ge, S., Mohamed, A. S., Wang, L., Li, Q., & Li, X. (2025). L-Theanine Metabolism in Tea Plants: Biological Functions and Stress Tolerance Mechanisms. Plants (Basel, Switzerland), 14(3), 492. https://doi.org/10.3390/plants14030492 
3. 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. https://doi.org/10.1179/147683010X12611460764840
4. Everett, J., Gunathilake, D., Dufficy, L., Roach, P., Thomas, J., Upton, D., & Naumovski, N. (2016). Theanine consumption, stress and anxiety in human clinical trials: A systematic review. Journal of Nutrition & Intermediary Metabolism, 4, 41–42. https://doi.org/10.1016/j.jnim.2015.12.308
5. Ritsner, M. S., Miodownik, C., Ratner, Y., Shleifer, T., Mar, M., Pintov, L., & Lerner, V. (2011). L-theanine relieves positive, activation, and anxiety symptoms in patients with schizophrenia and schizoaffective disorder: an 8-week, randomized, double-blind, placebo-controlled, 2-center study. The Journal of clinical psychiatry, 72(1), 34–42. https://doi.org/10.4088/JCP.09m05324gre
6. Williams, J., Kellett, J., Roach, P., McKune, A., Mellor, D., Thomas, J., & Naumovski, N. (2016). L-Theanine as a functional food additive: Its role in disease prevention and health promotion. Beverages, 2(2), 13. https://doi.org/10.3390/beverages2020013 
7. Chen, S., Kang, J., Zhu, H., Wang, K., Han, Z., Wang, L., Liu, J., Wu, Y., He, P., Tu, Y., & Li, B. (2023). L-Theanine and Immunity: A Review. Molecules (Basel, Switzerland), 28(9), 3846. https://doi.org/10.3390/molecules28093846
8. Miyachi, T., Tsuchiya, T., Oyama, A., Tsuchiya, T., Abe, N., Sato, A., Chiba, Y., Kurihara, S., Shibakusa, T., & Mikami, T. (2013). Perioperative oral administration of cystine and theanine enhances recovery after distal gastrectomy: a prospective randomized trial. JPEN. Journal of parenteral and enteral nutrition, 37(3), 384–391. https://doi.org/10.1177/0148607112458798
9. Li, C., Cai, R., Chen, L., Xu, S., Yang, X., & Huang, Y. (2024). L-theanine mechanism and application in the treatment of breast cancer. Theoretical and Natural Science, 29(1), 136–140. https://doi.org/10.54254/2753-8818/29/20240762 
10. Khan, N., & Mukhtar, H. (2013). Tea and health: studies in humans. Current pharmaceutical design, 19(34), 6141–6147. https://doi.org/10.2174/1381612811319340008
11. 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. https://doi.org/10.1186/1880-6805-31-28
12. Golden, E. B., Lam, P. Y., Kardosh, A., Gaffney, K. J., Cadenas, E., Louie, S. G., Petasis, N. A., Chen, T. C., & Schönthal, A. H. (2009). Green tea polyphenols block the anticancer effects of bortezomib and other boronic acid-based proteasome inhibitors. Blood, 113(23), 5927–5937. https://doi.org/10.1182/blood-2008-07-171389