L-theanine supplements


  • Vitamins & Supplements
  • Updated January 20, 2017

L-theanine is an amino acid structurally similar to glutamate and GABA, two neurotransmitters? important for brain function. It is contained in green, black, and oolong teas, which are all derived from Camellia sinensis, a perennial evergreen shrub. L-theanine is traditionally used to promote relaxation without sedative effects. Research suggests it is safe and has positive effects on cognitive function when combined with caffeine, but the effects of L-theanine alone appear to be modest and short-term. No studies have tested whether it can prevent dementia or cognitive decline.


Several clinical trials have examined the effects of L-theanine, alone or in combination with caffeine or green tea, on brain health. No human studies have tested it for the prevention of dementia or cognitive decline.

Our search identified:
• 1 meta-analysis based on 10 randomized controlled trials testing acute effects of L-theanine alone or in combination with caffeine in healthy adults
• 6 randomized controlled trials (3 testing acute effects, 2 examining brain activity, and 1 with green tea extract in people with mild cognitive impairment)
• 1 open-label study in people with major depressive disorder
• 1 review
• Numerous preclinical studies

Potential Benefit

The long-term effects of L-theanine on cognitive health are unknown, but a few short-term human studies have shown small benefits. In a randomized controlled trial in patients with mild cognitive impairment, L-theanine taken with green tea extract for 16 weeks had no significant effect on memory and attention, though a trend for an improvement in memory was seen midway through the trial [1]. The patients who had more severe impairment at the start of the trial appeared more likely to benefit from treatment, but these results have not yet been replicated.

Combinations of L-theanine and caffeine have been reported to acutely improve attention and alertness in small clinical trials, but the positive effects have been primarily attributed to caffeine [2]. L-theanine may interact with caffeine, improving attention and ability to ignore distractions, and together, enhance performance on cognitively demanding tasks [3]. In a small randomized controlled trial of healthy adults, L-theanine alone did not affect attentional focus [4]. Other studies have reported mixed effects, with L-theanine decreasing the beneficial effects of caffeine on cognition and mood [5].

Preclinical studies have reported that L-theanine may reduce brain cell death [6] and oxidative damage [7], and increase levels of protective chemicals [8], but these effects have not been confirmed in humans.

For Dementia Patients

It is currently unknown whether L-theanine can improve cognition or slow cognitive decline in people with dementia. While some benefits have been observed in preclinical studies of Alzheimer's disease [6], L-theanine has not been clinically studied in dementia patients.


Large meta-analyses of long-term tea consumption (containing L-theanine) have found that side effects are mild [9][10] but these results on tea may not apply to supplements. In a short randomized controlled trial, an L-theanine supplement (400 mg/day) was well-tolerated with no significant adverse events [11]. Larger, long-term studies are needed. For more information on green tea, please review our separate report.

The interactions between L-theanine and other drugs have not been well-studied. Clinical data suggest that L-theanine lessens the blood pressure increase caused by caffeine [12][13]. Because of its possible effects on blood pressure [12][14], it may be dangerous to use in combination with blood pressure medications.

NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.

How to Use

L-theanine supplements are available in both capsule and powder forms. A single cup (200 ml) of tea can contain 5 to 85 mg of L-theanine depending on the type, quality, and preparation of tea. Clinical trials examining the effects of L-theanine on cognitive function have used doses ranging from 12–250 mg/day, with the majority of studies using 200 mg/day [2][15][16].

Learn More

An analysis of commercially available products labeled as L-theanine.

United States Pharmacopeial Convention (USP) and FDA Information on Dietary Supplements offer information on the quality of specific supplements and can assist in finding a trusted brand.


  1. Park SK, Jung IC, Lee WK et al. (2011) A combination of green tea extract and l-theanine improves memory and attention in subjects with mild cognitive impairment: a double-blind placebo-controlled study. J Med Food 14, 334-343.
  2. Camfield DA, Stough C, Farrimond J et al. (2014) Acute effects of tea constituents L-theanine, caffeine, and epigallocatechin gallate on cognitive function and mood: a systematic review and meta-analysis. Nutr Rev 72, 507-522.
  3. Owen GN, Parnell H, De Bruin EA et al. (2008) The combined effects of L-theanine and caffeine on cognitive performance and mood. Nutr Neurosci 11, 193-198.
  4. Kelly SP, Gomez-Ramirez M, Montesi JL et al. (2008) L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance. J Nutr 138, 1572S-1577S.
  5. Dodd FL, Kennedy DO, Riby LM et al. (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 (Berl) 232, 2563-2576.
  6. Kim TI, Lee YK, Park SG et al. (2009) l-Theanine, an amino acid in green tea, attenuates beta-amyloid-induced cognitive dysfunction and neurotoxicity: reduction in oxidative damage and inactivation of ERK/p38 kinase and NF-kappaB pathways. Free Radic Biol Med 47, 1601-1610.
  7. Nishida K, Yasuda E, Nagasawa K et al. (2008) Altered levels of oxidation and phospholipase C isozyme expression in the brains of theanine-administered rats. Biol Pharm Bull 31, 857-860.
  8. Cho HS, Kim S, Lee SY et al. (2008) Protective effect of the green tea component, L-theanine on environmental toxins-induced neuronal cell death. Neurotoxicology 29, 656-662.
  9. Hartley L, Flowers N, Holmes J et al. (2013) Green and black tea for the primary prevention of cardiovascular disease. Cochrane Database Syst Rev, CD009934.
  10. Jurgens TM, Whelan AM, Killian L et al. (2012) Green tea for weight loss and weight maintenance in overweight or obese adults. Cochrane Database Syst Rev 12, CD008650.
  11. Lyon MR, Kapoor MP, Juneja LR (2011) The effects of L-theanine (Suntheanine(R)) on objective sleep quality in boys with attention deficit hyperactivity disorder (ADHD): a randomized, double-blind, placebo-controlled clinical trial. Altern Med Rev 16, 348-354.
  12. Rogers PJ, Smith JE, Heatherley SV et al. (2008) Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together. Psychopharmacology (Berl) 195, 569-577.
  13. Yoto A, Motoki M, Murao S et al. (2012) Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses. J Physiol Anthropol 31, 28.
  14. Siamwala JH, Dias PM, Majumder S et al. (2013) L-theanine promotes nitric oxide production in endothelial cells through eNOS phosphorylation. J Nutr Biochem 24, 595-605.
  15. Kahathuduwa CN, Dassanayake TL, Amarakoon AM et al. (2016) Acute effects of theanine, caffeine and theanine-caffeine combination on attention. Nutr Neurosci.
  16. White DJ, de Klerk S, Woods W et al. (2016) Anti-Stress, Behavioural and Magnetoencephalography Effects of an L-Theanine-Based Nutrient Drink: A Randomised, Double-Blind, Placebo-Controlled, Crossover Trial. Nutrients 8.