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Short answer: L-theanine appears to be well tolerated. In animal safety studies, no adverse effects were seen even at very high doses, and in a 4-week human trial of 200 mg a day, no significant adverse events were reported. It does not appear to act as a sedative at the doses used for relaxation, though one study found it slightly slowed reaction time. It may blunt rises in blood pressure, so check with your provider if you take blood pressure medicine. Memorial Sloan Kettering lists pregnancy and breastfeeding, sedative medicines and green tea allergy as reasons to avoid it. Long-term human data are limited.
L-theanine is often described as gentle, and for most people it seems to be. But "natural" does not automatically mean side-effect free, and supplement doses are much higher than what you get from a cup of tea. Here is what safety studies show, the side effects that have been reported, possible effects on blood pressure and the heart, and who should check with a provider first. For an overview of what L-theanine is and does, see what is L-theanine?
What you'll learn
- What animal and human safety studies show
- Side effects that have been reported
- L-theanine, blood pressure and the heart
- Who should avoid L-theanine or check with a provider first
Is L-theanine safe?
Based on current evidence, L-theanine appears to have a good safety profile. In rats, a 13-week study following international testing guidelines found no consistent treatment-related adverse effects at doses up to 4,000 mg per kg of body weight a day, the highest dose tested (Borzelleca et al., 2006). Other animal studies found no toxicity in shorter tests at higher doses and no toxic or cancer-causing effects when L-theanine made up 5% of the diet for 78 weeks, and it showed no genetic toxicity in a standard lab test (Dashwood & Visioli, 2025).
Animal results do not translate directly to people, and human data are thinner. In a 4-week trial in 30 healthy adults taking 200 mg a day, there were no drop-outs and no significant adverse events (Hidese et al., 2019). Most human studies have been small and short, so long-term safety at supplement doses is not well established (Dashwood & Visioli, 2025).
Reported side effects
| Possible effect | What studies show |
|---|---|
| Drowsiness | Doses used to promote relaxation did not have hypnotic or sedative side effects (Dashwood & Visioli, 2025). |
| Slower reaction time | In one study of 48 adults, 200 mg of L-theanine slowed overall reaction time on an attention task (Rogers et al., 2008). |
| Lower blood pressure response | L-theanine blunted blood pressure rises from caffeine or stress in small studies (Rogers et al., 2008; Yoto et al., 2012). See below. |
| General side effects | No significant adverse events in a 4-week trial of 200 mg a day (Hidese et al., 2019). |
Because studies are small and many come from one commercial source of L-theanine (Dashwood & Visioli, 2025), less common side effects could be missed. If you notice anything unusual after starting a supplement, stop and talk with your provider.
L-theanine, blood pressure and the heart
Most research on the heart looks at blood pressure. In a study of 48 healthy adults, 250 mg of caffeine raised blood pressure, and 200 mg of L-theanine counteracted that rise (Rogers et al., 2008). In another small study, L-theanine reduced the rise in blood pressure during stressful mental tasks in people whose blood pressure tended to jump the most (Yoto et al., 2012).
For most people, that is not a side effect to worry about. But if you take medicine for high blood pressure, or have low blood pressure, ask your provider before adding an L-theanine supplement. For more on caffeine and blood pressure, see does caffeine raise blood pressure?
Who should avoid L-theanine or ask first
Memorial Sloan Kettering Cancer Center's herb database lists these contraindications (Memorial Sloan Kettering):
- Pregnancy and breastfeeding. Pregnant women are also usually excluded from L-theanine trials (Hidese et al., 2019).
- Taking sedatives. In animal studies, L-theanine had additive effects with the sedative midazolam; how much this matters in people is not known (Memorial Sloan Kettering).
- Allergy or sensitivity to green tea.
Also talk with your provider if you take blood pressure medicine, other medicines that affect the brain, or several supplements at once.
How much is too much?
We could not find an official upper limit for L-theanine in people. Trials for stress have typically used 200 to 400 mg a day (Williams et al., 2020), while a cup of black tea has about 20 mg (Nobre et al., 2008). Reviewers have urged caution about taking supplements at high, drug-like doses until better human trials are done (Dashwood & Visioli, 2025). The practical approach: follow the label, start low, and do not stack several calming supplements without advice.
L-theanine from tea vs supplements
Getting L-theanine from tea or matcha means much smaller amounts, along with caffeine and other tea compounds. It is important to distinguish L-theanine as a natural part of tea from its use as a supplement at higher doses (Dashwood & Visioli, 2025). If you prefer to get it from food, see does matcha have caffeine? and matcha side effects and how much is safe.
Supplement quality and claims
L-theanine supplements vary in quality and dose. In Europe, regulators did not approve a health claim linking L-theanine to attention, and independent reviewers concluded that the data are promising but far too preliminary to support many of the claims made for it (Dashwood & Visioli, 2025). Be wary of products that make medical-sounding promises.
Common myths (quick reality check)
Myth: L-theanine makes you drowsy. Reality: At the doses used for relaxation, L-theanine did not have sedative side effects in research.
Myth: Natural means no side effects. Reality: L-theanine appears well tolerated, but long-term human data are limited, and it may interact with sedatives and affect blood pressure.
Myth: Dose does not matter with L-theanine. Reality: No official upper limit has been set that we could find, and reviewers urge caution with high supplement doses.
Frequently asked questions (FAQ)
What are the side effects of L-theanine?
L-theanine appears well tolerated. One study found it slightly slowed reaction time, and it may blunt rises in blood pressure. No significant adverse events were seen in a 4-week trial.
Does L-theanine affect your heart?
Research mostly looks at blood pressure: L-theanine reduced blood pressure rises from caffeine or stress in small studies. Ask your provider if you take blood pressure medicine.
Can L-theanine make you tired?
Not usually. At relaxation doses, it did not show sedative side effects, but it may combine with sedative medicines.
Is it safe to take L-theanine every day?
Short-term daily use of 200 mg showed no significant adverse events in one trial, but long-term data are limited. Check with your provider.
Who should not take L-theanine?
People who are pregnant or breastfeeding, those taking sedatives and those allergic to green tea should avoid it or ask a provider first.
How much L-theanine is too much?
No official upper limit has been set that we could find. Studies typically used 200 to 400 mg a day. Follow the label and avoid high doses without advice.
Related guides
- What is L-theanine?
- L-theanine and caffeine
- Is matcha bad for you?
- Alpha GPC side effects and safety
- Does caffeine raise blood pressure?
Disclaimer and when to seek help
This article is for general education and is not medical advice. These statements have not been evaluated by the Food and Drug Administration. Our products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your health care provider before taking L-theanine, especially if you are pregnant or breastfeeding, take sedatives, blood pressure or other medication, or have a medical or mental health condition.
References
- Dashwood R, Visioli F. "l-theanine: From tea leaf to trending supplement - does the science match the hype for brain health and relaxation?" Nutrition Research, 2025;134:39-48. doi:10.1016/j.nutres.2024.12.008
- Borzelleca JF, Peters D, Hall W. "A 13-week dietary toxicity and toxicokinetic study with l-theanine in rats." Food and Chemical Toxicology, 2006;44(7):1158-1166. doi:10.1016/j.fct.2006.03.014
- Hidese S, et al. "Effects of L-Theanine Administration on Stress-Related Symptoms and Cognitive Functions in Healthy Adults: A Randomized Controlled Trial." Nutrients, 2019;11(10):2362. doi:10.3390/nu11102362
- Rogers PJ, et al. "Time for tea: mood, blood pressure and cognitive performance effects of caffeine and theanine administered alone and together." Psychopharmacology, 2008;195(4):569-577. doi:10.1007/s00213-007-0938-1
- Yoto A, et al. "Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses." Journal of Physiological Anthropology, 2012;31:28. doi:10.1186/1880-6805-31-28
- Memorial Sloan Kettering Cancer Center. "L-Theanine" About Herbs, Botanicals and Other Products. Last updated July 14, 2026.
- 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 for Human Nutrition, 2020;75(1):12-23. doi:10.1007/s11130-019-00771-5
- Nobre AC, Rao A, Owen GN. "L-theanine, a natural constituent in tea, and its effect on mental state." Asia Pacific Journal of Clinical Nutrition, 2008;17 Suppl 1:167-168. PubMed 18296328
