Product Reviews
L-Theanine Supplements Review

Initial Posting: 8/8/15  Last Update: 5/12/20L-Theanine Supplements reviewed by

Sections: Jump to a section by clicking on its name.
  • Often just called theanine, L-theanine, is a water-soluble amino acid found in black and green tea. (See What It Is).
  • Some small clinical studies (funded by manufacturers of L-theanine) have found L-theanine may reduce stress, anxiety and blood pressure in stressful situations, but not all studies have found a benefit (See What It Does).
  • L-theanine is typically taken at a dose of 200 mg to 400 mg per day (See Dosage).
  • At these doses, it appears to be generally safe, but there are possible interactions with blood pressure medications and benzodiazepines (see Concerns and Cautions).
  • did not find problems with the quality of selected L-theanine products sold in the U.S., but did find large differences in the cost to obtain L-theanine. The least expensive, Approved product cost 22 cents to obtain 200 mg of L-theanine while the cost was often 60 cents for other products (See What CL Found).

What It Is:
L-theanine is a water-soluble amino acid found primarily in black and green tea, and certain mushrooms. The L-theanine in supplements, however, is generally synthetic, produced from food grade L-glutamine and ethylamine, but may also be an enzymatically-processed extract of tea leaves. It is thought to have both antioxidant and relaxant effects, possibly through influence on excitatory chemicals in the body, such as noradrenaline, and chemical messengers in the brain, such as GABA and serotonin.

What It Does:
There is mixed evidence as to whether L-theanine reduces stress and anxiety. For example, a placebo-controlled study showed that taking 50 mg of L-theanine increased alpha brain waves, which are associated with a relaxed but alert state of mind (Nobre, Asia Pac Clin Nutr 2008). The effect was measured 45 minutes after L-theanine was taken and continued to increase over the 1½ hours that brain waves were measured. One small placebo-controlled clinical trial found that 200 mg L-theanine (Suntheanine) dissolved in water and taken after beginning a stressful cognitive task significantly reduced self-reported anxiety and physiological measures of stress, such as increased heart rate (Kimura, Biol Psychol 2007).

A study among middle-aged adults in Japan with mild emotional, sleep, and/or cognitive symptoms found that a 200-mg tablet of L-theanine taken before sleep each night for four weeks led to improvements from baseline on most measures. However, compared to results with placebo, these improvements were only statistically significant for three measures: time to fall asleep, sleep efficiency and reduced sleep disturbances, and were only significant for the later two measures because of adverse results in the placebo group. L-theanine did not result in any statistically significant improvement relative to placebo on cognitive function tests (Hidese, Nutrients 2019).

Another study using the same dose and brand of L-theanine found that it did not reduce anxiety when taken by young adults prior to a stressful task — although neither did 1 mg of alprazolam (Xanax). Interestingly, during a non-stressful task, treatment with the alprazolam was found to significantly increase self-reported anxiety while L-theanine significantly decreased self-reported anxiety (Lu, Hum Psychopharmacol Clin Exp 2004). A small study among men and women in Australia with generalized anxiety disorder (GAD) (many of whom also had social phobia, panic disorder or agoraphobia), found that daily supplementation with L-theanine for two months (450 mg daily for one month, increased to 900 mg for the second month for those who did not respond) did not decrease anxiety, or improve cognition or insomnia severity compared to placebo, although those taking L-theanine reported improved "sleep satisfaction." (Sarris, J Psychiatr Res 2019).

At higher doses, L-theanine may be helpful for people with schizophrenia. In an independent study of people diagnosed with schizophrenia or schizoaffective disorder taking antipsychotic medication, 400 mg of L-theanine taken daily for 8 weeks significantly reduced measures of anxiety and other symptoms, compared to placebo (Ritsner, J Clin Psychiatry 2011). L-theanine has been suggested as a treatment in ADHD (attention deficit hyperactivity disorder), although the evidence of benefit is very limited. In a 10-week study of 93 boys (average age 9) diagnosed with ADHD, some of whom were also taking stimulant medication such as methylphenidate (Ritalin), 200 mg of L-theanine in the morning and again in the afternoon (total daily intake of 400 mg) was found to significantly improve sleep quality compared to placebo, although it did not reduce the amount of time it took to fall asleep. This study did not measure effects on daytime hyperactivity, attention, or cognitive function (Lyon, Altern Med Rev 2011).

All of the studies (other than the study by Ritsner) used a branded version of L-theanine called Suntheanine from Taiyo International, Japan, which typically funded the studies. (Suntheanine is the form of L-theanine found in most of the products in this Review, as shown in the second column of the Results table below).

L-Theanine may lessen blood pressure increases associated with caffeine use. One study found that healthy adults who consumed a drink containing 200 mg of L-theanine (provided by Unilever plc, UK) in addition to taking a capsule containing 250 mg of caffeine had less of an increase in blood pressure than those who consumed only caffeine. It did not, however, reduce other effects of the caffeine, such as alertness or jitteriness (Rogers, Psychopharmacology 2008).

Although some laboratory and animal studies suggest L-theanine may have anti-tumor properties, there is a lack of human studies for this use. Similarly, while animal studies suggest L-theanine may enhance the effects and/or reduce the side-effects of certain chemotherapy drugs, such as doxorubicin and adriamycin, human studies have not been conducted (Sugiyama, Cancer Lett 1998, Sugiyama, Cancer Lett 2004;).

Quality Concerns and What CL Tested for:
Neither the Food and Drug Administration (FDA) nor any other federal or state agency routinely tests supplements for quality prior to sale. In order to help consumers identify products of better quality, purchased and tested L-theanine supplements to determine whether they
contained the labeled amounts of L-theanine. In addition, tablets were tested for their ability to properly break apart (disintegrate) in solution. Products were also tested for potential contamination with lead, cadmium, and arsenic. (See How Products Were Evaluated for information on testing methods and passing score.)

Join |  Sign In
Join Us on Facebook! Join Us on Instagram! Join Us on Twitter! Join Us on YouTube! Join Us on YouTube!
Product Reviews
Brands Tested
Health Conditions
CL Answers
Clinical Updates
Recalls & Warnings
Recommended Intakes
Where to Buy Products
Testing Program
How Products Were Tested
Quality Certification Program
Join CL Today
Join Free Newsletter
Group Subscriptions
Gift Membership
About Us
The CL Seal
CL Survey
Privacy Policy
Contact Us/Help

©2020, LLC. All rights reserved. A single copy of a report may be printed for personal use by the subscriber. It is otherwise unlawful to print, download, store or distribute content from this site without permission. name and flask logo are both registered trademarks of, LLC. This site is intended for informational purposes only and not to provide medical advice.
Join our FREE Newsletter and Become a Member to View
L-Theanine Supplements Review!
First Name 
Last Name 
Retype Email* Member Benefits:

Instant Access to All Product Review Reports Covering Over 1,000 Products
Quality Ratings and Product Comparisons by Brand
Expert Tips on Using Supplements
Membership fee required for full benefits.
Price Checks on Popular Brands
e-Newsletter with Updates and Alerts
New and Archived Recalls and Warnings