L-theanine side effects: what is known and who should be careful?

Short answer: Studies using 100 to 400 mg L-theanine a day reported no serious side effects. Those studies were small, though, and lasted 8 weeks at most, so little is known about long-term use. For our capsules: do not use if you are pregnant, breastfeeding or under 18, and talk to your doctor first if you take medication.

Last updated: 6 November 2026 · Written by Florian Fermin

Below you can read which L-theanine side effects and interactions have been reported in research, what is still unknown and why our label carries warnings for green tea extract. What L-theanine is, you can read in L-theanine effects: what research says about dosage and use. We at GOYU sell L-theanine capsules ourselves, so I would rather be too careful than too sure.

Does L-theanine have side effects?

No serious side effects have been reported in studies. That does not mean L-theanine is risk-free for everyone: most studies were small, short and done in healthy adults.

• A meta-analysis of 31 randomised studies with 1,168 participants in total compared how many people dropped out and why. No serious adverse events were reported (Gerolymos et al., 2026).
• In a Japanese study, 30 healthy adults took 200 mg a day for 4 weeks. Nobody dropped out and the researchers saw no adverse events (Hidese et al., 2019). Two authors work for an L-theanine manufacturer.
• In an animal study, rats were given up to 4,000 mg L-theanine per kilo of body weight a day for 13 weeks. At the highest dose, the researchers saw no harmful effect attributable to L-theanine (Borzelleca et al., 2006). That is far more than in a supplement, but animal studies do not tell the whole story for humans.

Which side effects are mentioned?

Clear, recurring side effects are not described in the studies we read. There are a few things worth knowing:

If you get complaints after starting, take less or stop, and see your doctor if they persist.

What interactions does L-theanine have?

There are no good studies on interactions with medicines. That is why our label says: consult your doctor before use if you take medication or have a medical condition. These are the combinations I would pay extra attention to:

• Caffeine: L-theanine and caffeine have often been studied together. One study measured what 250 mg caffeine and 200 mg L-theanine, alone and together, did to blood pressure (Rogers et al., 2008). Watch your caffeine above all: EFSA sees no safety concern for healthy adults up to 400 mg a day from all sources, and up to 200 mg at once (EFSA, 2015).
• Blood pressure medication: because L-theanine has been studied in relation to blood pressure, check with your doctor or pharmacist first.
• Medicines that act on your nervous system, such as sleeping pills and sedatives: L-theanine is mainly studied for brain activity and reaction time. Check first.
• Other products containing green tea: according to the pack, you should not use our capsules on a day when you also take other products containing green tea, such as matcha or green tea. You can read why below. How much L-theanine tea contains is covered in L-theanine in green tea.

Why does the label carry warnings for green tea extract?

According to the label, our capsules contain green tea extract. In the EU, green tea extract containing epigallocatechin gallate (EGCG), a compound from the tea leaf, must carry set warnings on the label. Those warnings are about EGCG, not about L-theanine itself.

In 2018, the European Food Safety Authority (EFSA) concluded that 800 mg EGCG or more a day from supplements was linked to raised liver values in the blood (EFSA, 2018). That is why, under Regulation (EU) 2022/2340, a daily portion must contain less than 800 mg EGCG and the label must include these warnings:

• “Should not be consumed if you are consuming other products containing green tea on the same day.”
• “Should not be consumed by pregnant and lactating women and children below 18 years of age.”
• “Should not be consumed on an empty stomach.”

Our extract is standardised to 98% L-theanine: 200 mg extract with 196 mg L-theanine per capsule. We still apply these warnings to our capsules and advise you to do the same. In the French government database for food supplements, the leaf of Camellia sinensis is listed as permitted with exactly these warnings, and theanine as a permitted substance (Compl'Alim).

Who should not take L-theanine?

Do not use our L-theanine capsules if you are pregnant, breastfeeding or under 18. This is listed with the warnings on our product page. Also:

• Medication or a medical condition: consult your doctor before use.
• Complaints that may point to a liver problem: stop using it and consult a doctor. This is also on our product page.
• You already use matcha, green tea or another green tea product: choose one of the two per day.
• You need to stay sharp, for example in traffic: try L-theanine first at a time when you do not need to drive.

Do not exceed the daily dose: 1 capsule, once or twice a day, preferably with or just after a meal. How much and when exactly is covered in L-theanine dosage.

Does it matter where the L-theanine comes from?

Yes, for its legal status. The European Commission's Novel Food Catalogue states that L-theanine extracted and isolated from green tea is not a novel food in food supplements, and only there. L-theanine from other sources, for example made by fermentation, is a novel food and may only be sold once its safety has been assessed and it has been authorised (Novel Food Catalogue). In the French database Compl'Alim, theanine is listed as a permitted substance and not as a novel food.

Summary in 3 points

• Studies using 100 to 400 mg a day reported no serious side effects, but they were small and short. Little is known about use beyond 8 weeks.
• Good studies on interactions are lacking. Check first if you take medication, such as blood pressure medicines or medicines that act on your nervous system.
• We apply the EU warnings for green tea extract to our capsules: not when pregnant, not when breastfeeding, not under 18, not on an empty stomach and not on the same day as other products containing green tea.

Frequently asked questions

Is L-theanine harmful?

Studies using 100 to 400 mg a day for up to 8 weeks reported no serious side effects. Little is known about longer use or use alongside medication. Stick to the daily dose and talk to your doctor if in doubt.

Does L-theanine make you drowsy?

That is often mentioned online, but drowsiness was not measured systematically in the studies we read. In one study, people reacted slightly more slowly on a computer task after 200 mg. If you notice you feel drowsy, do not drive.

Can you combine L-theanine with coffee?

Many studies did, usually with 50 to 250 mg caffeine. Watch your total caffeine: EFSA uses 400 mg a day for healthy adults. Coffee is not covered by the green tea warning; matcha and green tea are.

Can you take L-theanine when pregnant?

No, not our capsules: our product page says they are not intended for pregnant women, breastfeeding women or children under 18. Talk to your midwife or doctor about tea and caffeine during pregnancy.

Does L-theanine interact with medication?

That has not been studied well. Because L-theanine has been studied in relation to blood pressure and brain activity, always check with your doctor or pharmacist first if you take blood pressure medicines or medicines that act on your nervous system.

Sources

1. Gerolymos C, Saddier E, Boyer L, Fond G (2026). Molecular Psychiatry 31(11):6592-6602. Meta-analysis of 31 randomised studies, safety assessed via dropouts and reasons for dropout. doi:10.1038/s41380-026-03727-9
2. Hidese S, Ogawa S, Ota M, et al. (2019). Nutrients 11(10):2362. Randomised, double-blind, crossover, 30 adults, 4 weeks; two authors work for an L-theanine manufacturer. doi:10.3390/nu11102362
3. Borzelleca JF, Peters D, Hall W (2006). Food and Chemical Toxicology 44(7):1158-1166. 13-week toxicity study in rats. doi:10.1016/j.fct.2006.03.014
4. Rogers PJ, Smith JE, Heatherley SV, Pleydell-Pearce CW (2008). Psychopharmacology 195(4):569-577. 48 adults, single dose of L-theanine and caffeine. doi:10.1007/s00213-007-0938-1
5. Yoto A, Motoki M, Murao S, Yokogoshi H (2012). Journal of Physiological Anthropology 31:28. 14 participants, crossover. doi:10.1186/1880-6805-31-28
6. Williams JL, Everett JM, D'Cunha NM, et al. (2020). Plant Foods for Human Nutrition 75(1):12-23. Systematic review of 9 randomised studies. PubMed 31758301
7. EFSA NDA Panel (2015). Scientific Opinion on the safety of caffeine. EFSA Journal 13(5):4102. doi:10.2903/j.efsa.2015.4102
8. EFSA ANS Panel (2018). Scientific opinion on the safety of green tea catechins. EFSA Journal 16(4):5239. doi:10.2903/j.efsa.2018.5239
9. Commission Regulation (EU) 2022/2340 on green tea extracts containing (-)-epigallocatechin-3-gallate. eur-lex.europa.eu
10. Compl'Alim (French government), entries Camellia sinensis (L.) Kuntze and théanine, consulted 6 November 2026. compl-alim.beta.gouv.fr
11. European Commission, Novel Food Catalogue, entry L-theanine (POL-NF-00443), consulted November 2026. food.ec.europa.eu
12. GOYU, label and product page Green Tea Extract Capsules 200 mg (L-Theanine), consulted 6 November 2026.

Products with a claim related to focus and concentration, such as matcha and ginkgo, are at supplements for focus and concentration. Our L-theanine capsules are not listed there, because no claim has been approved for L-theanine.

This article is for information only and is not medical advice. Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. If in doubt, pregnant or taking medication, talk to your doctor or pharmacist.