L-Theanine Side Effects, Interactions and Who Should Ask First
What trials report about L-theanine side effects, what is unknown about medicine interactions, blood pressure and pregnancy, and who should ask a clinician or pharmacist first.
Content last updated . This page has not been medically reviewed.
Quick answer
In the human trials we read, L-theanine was reported as well tolerated, but the trials were small and short, and for several important questions, Proper evidence not found. We searched PubMed for theanine with safety, adverse events, pregnancy, lactation and blood pressure. Not finding evidence of harm is not proof of safety, and "natural" does not mean safe for everyone. This page is information only. A clinician or pharmacist can judge your own situation.
How the evidence is labeled
Reported in trials means a study measured and reported it, usually in small groups over weeks. Preclinical means animals or cells, which cannot show what happens in people. Insufficient evidence means too little to judge.What the sources found
| Question | What the source found | Strength of evidence |
|---|---|---|
| Side effects in children | In 98 boys with ADHD taking L-theanine for six weeks, the authors reported it was well tolerated with no significant adverse events. [1] | Reported in one trial |
| Side effects in adults | In a 28-day trial of 30 adults, all adverse events resolved by the end of the study or at follow-up, and lab and vital-sign changes were judged not clinically relevant. [2] | Reported in one small trial |
| Overall safety reviews | A review of toxicity tests concluded L-theanine is "reliable in general," but also said clear evidence-based results on its metabolism, health effects and safety had not yet been shown. [3] | Limited |
| Long-term use | Proper evidence not found in the trials we read. They lasted days to a few weeks, and a stress review called for longer studies. [4] | Insufficient evidence |
| Blood pressure | A 2026 review describes blood-pressure effects within "growing preclinical evidence and pending clinical validation." [5] We did not find a trial showing how it affects people who take blood-pressure medicine. | Preclinical; insufficient evidence in people |
| Sedatives and other nervous-system medicines | In mice, theanine changed the response to a barbiturate sedative and to strychnine, so the authors concluded it interacted with them. [6] We found no human interaction studies. | Preclinical |
| Pregnancy and breastfeeding | Proper evidence not found. A PubMed search for theanine with pregnancy and lactation returned no human safety study. | None |
| Kidney and liver conditions | Proper evidence not found for L-theanine itself. | None |
| Before surgery | Not described in the sources we read. | None |
Do not borrow results from green tea
Green tea and green tea extracts contain other things, including caffeine. The NCCIH says no safety concerns have been reported for green tea as a drink for adults, but that extract supplements can cause nausea, constipation, abdominal discomfort and higher blood pressure, and that liver injury has been reported in some people, mainly with extracts in tablet or capsule form. [7] It also reports that green tea extract can lower blood levels of a cholesterol medicine. [7] Those findings are about green tea products, not isolated L-theanine, and should not be assumed to apply to it, or the other way round. See Green Tea.Caffeine in tea and in combined products
Many studies pair L-theanine with caffeine, and tea contains both. Caffeine has its own effects, from jitters to a faster heartbeat. [8] If a product has caffeine, check Caffeine and Medicines and the pregnancy guidance in Caffeine in Pregnancy, Breastfeeding, Children and Teens. The NCCIH advises anyone who is or might be pregnant to ask a provider about caffeine from green tea. [7]Product quality
In the US, the FDA does not have the authority to approve dietary supplements' safety, effectiveness or labeling before they go on the market, according to a 2023 review. [9] Contents and strength can vary between products. See Quality Testing and Certifications.Who should ask first
Because human interaction and long-term data are missing, it is reasonable to ask a pharmacist or clinician before using L-theanine if you:- take prescribed medicines, especially for blood pressure, sleep, anxiety or mood, or other medicines that act on the nervous system;
- are pregnant, trying to become pregnant or breastfeeding;
- are giving it to a child or teen;
- have a liver or kidney condition;
- have surgery or a procedure planned;
- have ADHD, an anxiety disorder, a sleep disorder or another condition under care. Do not stop or change a prescribed medicine on your own. See L-Theanine for Attention and ADHD.
Tell the pharmacist exactly what is on the product label, including any other ingredients. See also Medication Interactions and Pregnancy and Breastfeeding.
Where to go next
L-Theanine: What It Is, Evidence and Safety · L-Theanine for Sleep, Stress and Anxiety · Nootropic Safety and Claim ChecksFrequently asked questions
What are the side effects of L-theanine?
In the trials we read, the authors reported no significant adverse events in boys over six weeks, and adverse events that resolved in a 28-day adult trial. [1][2] Trials were small and short.
Does L-theanine lower blood pressure?
We did not find a human trial showing how it affects people on blood-pressure medicine. A 2026 review links blood-pressure effects to preclinical evidence with clinical validation pending. [5] Ask a pharmacist if you take blood-pressure medicine.
Is L-theanine safe in pregnancy?
Proper evidence not found. We searched PubMed for theanine with pregnancy and breastfeeding and found no human safety study. Ask a clinician. If the product is tea or contains caffeine, ask about caffeine too. [7]
Can I take L-theanine with sleep or anxiety medicine?
No human interaction study was found. In mice, theanine changed the effect of a barbiturate sedative. [6] That is preclinical, but it is a reason to ask a pharmacist before combining.
Is L-theanine from tea safer than a supplement?
The NCCIH reports no safety concerns for green tea as a drink for adults, with the caveat that it contains caffeine, while extract supplements have reported side effects. [7] That is about green tea, not isolated L-theanine.
References
- Lyon MR et al., The effects of L-theanine on objective sleep quality in boys with attention deficit hyperactivity disorder (ADHD): a randomized, double-blind, placebo-controlled clinical trial. Altern Med Rev 2011. Read 2026-10-23.
- Moulin M et al., Safety and efficacy of L-theanine supplementation for 28 days in healthy adults with moderate stress: a randomized, double-blind, placebo-controlled trial. Neurol Ther 2024. Read 2026-10-23.
- Türközü D, Şanlier N, L-theanine, unique amino acid of tea, and its metabolism, health effects, and safety. Crit Rev Food Sci Nutr 2017. Read 2026-10-23.
- Williams JL et al., The effects of green tea amino acid L-theanine consumption on the ability to manage stress and anxiety levels: a systematic review. Plant Foods Hum Nutr 2020. Read 2026-10-23.
- García-Niño WR et al., Pharmacological effects and molecular targets of L-theanine in cardiovascular diseases and comorbidities. Eur J Pharmacol 2026 (review). Read 2026-10-23.
- Yu XC et al., Theanine enhanced both the toxicity of strychnine and anticonvulsion of pentobarbital sodium. Drug Chem Toxicol 2016 (mouse study). Read 2026-10-23.
- National Center for Complementary and Integrative Health (US), Green Tea (last updated February 2025) Read 2026-10-23.
- U.S. Food and Drug Administration (US), Spilling the Beans: How Much Caffeine is Too Much? (content current as of 08/28/2024) Read 2026-10-23.
- Hersant H et al., Over the counter supplements for memory: a review of available evidence. CNS Drugs 2023. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.