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L-Theanine Side Effects & Safety

Evidence:Moderate
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This content is for informational purposes only and does not constitute medical advice. Statements about dietary supplements have not been evaluated by the FDA and are not intended to diagnose, treat, cure, or prevent any disease. Individual results may vary — consult your healthcare provider before starting any supplement. Full disclaimer

Safety Profile

Overall safety rating: Generally Safe

Potential Side Effects

  • Extremely well tolerated — virtually no side effects at standard doses
  • Mild drowsiness at very high doses in some individuals
  • Very slight blood pressure reduction

Drug & Supplement Interactions

  • Caffeine — synergistic (beneficial interaction): enhances focus while reducing jitteriness
  • Blood pressure medications — mild additive hypotensive effect
  • Stimulant medications — may modulate effects; generally complementary

Maximum Dose

Do not exceed: 400 mg/day is commonly used; up to 900 mg/day has been studied safely

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References

  1. RCTNobre AC, Rao A, Owen GN (2008). L-theanine, a natural constituent in tea, and its effect on mental state. Asia Pacific Journal of Clinical Nutrition. PubMed
  2. RCTOwen GN, Parnell H, De Bruin EA, Rycroft JA (2008). The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutritional Neuroscience. DOI PubMed
  3. Meta-analysisBulman A, D'Cunha NM, Marx W, Turner M, et al. (2025). The effects of L-theanine consumption on sleep outcomes: A systematic review and meta-analysis.. Sleep medicine reviews. DOI PubMed
  4. Payne ER, Aceves-Martins M, Dubost J, Greyling A, et al. (2025). Effects of Tea (Camellia sinensis) or its Bioactive Compounds l-Theanine or l-Theanine plus Caffeine on Cognition, Sleep, and Mood in Healthy Participants: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.. Nutrition reviews. DOI PubMed
  5. Camfield DA, Stough C, Farrimond J, Scholey AB (2014). Acute effects of tea constituents L-theanine, caffeine, and epigallocatechin gallate on cognitive function and mood: a systematic review and meta-analysis.. Nutrition reviews. DOI PubMed
  6. Cotter J, Caddick CE, Harper JL, Ebajemito JK (2026). Examining the effect of L-theanine on sleep: a systematic review of dietary supplementation trials.. Nutritional neuroscience. DOI PubMed
  7. Moshfeghinia R, Sanaei E, Mostafavi S, Assadian K, et al. (2024). The effects of L-theanine supplementation on the outcomes of patients with mental disorders: a systematic review.. BMC psychiatry. DOI PubMed
Show 5 more references
  1. ReviewWilliams JL, Everett JM, D'Cunha NM, Sergi D, 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 (Dordrecht, Netherlands). DOI PubMed
  2. Konno H, Naganuma K, Kamada Y (2026). Effects of combined GABA and L-theanine supplementation on sleep quality: an exploratory study.. European review for medical and pharmacological sciences. DOI PubMed
  3. Razazan R, Hemmatinafar M, Imanian B, Jahaniboushehri N, et al. (2025). Performance-enhancing effects of caffeine and L-Theanine among Iranian elite wrestlers: a focus on cognitive and specific physical performance.. Journal of the International Society of Sports Nutrition. DOI PubMed
  4. RCTMoulin M, Crowley DC, Xiong L, Guthrie N, et al. (2024). Safety and Efficacy of AlphaWave® L-Theanine Supplementation for 28 Days in Healthy Adults with Moderate Stress: A Randomized, Double-Blind, Placebo-Controlled Trial.. Neurology and therapy. DOI PubMed
  5. McAllister MJ, Martaindale MH, Dillard CC, McCullough R (2024). Impact of L-theanine and L-tyrosine on markers of stress and cognitive performance in response to a virtual reality based active shooter training drill.. Stress (Amsterdam, Netherlands). DOI PubMed