L-Theanine Side Effects, Safety and How Long It Lasts

L-theanine side effects in randomized trials have been few and mild, and a 2026 meta-analysis of 31 trials reported no serious side effects. On heart rate, studies have not found L-theanine speeding up the heart: in small stress studies it eased the rise in heart rate, and in a small trial at rest, heart rate did not change.

This guide covers what randomized trials have reported about L-theanine safety, what the research shows about your heart and blood pressure, how long this calming amino acid from tea lasts and stays in your system, how much has been studied, and who should check with a clinician first.

It is educational, not medical advice.

Key Takeaways

• Side effects: a 2026 meta-analysis of 31 randomized trials (1,168 people) reported no serious side effects with L-theanine.
• Amounts studied: trials have used 50 to 1,000 mg a day, mostly in the 100 to 400 mg range, for up to eight weeks.
• Heart: in small studies, L-theanine eased the rise in heart rate and blood pressure during a stressful task.
• How long it lasts: blood levels peak about 45 to 50 minutes after a dose, and its half-life is about 1 hour.
• Not a sedative: reviews report it does not appear to cause sedation.

L-Theanine Side Effects: What Trials Report

L-theanine is an amino acid found in tea leaves. It has been tested in dozens of randomized, placebo-controlled trials, and the safety picture across them is consistent:

• 31 trials, 1,168 people (2026). A meta-analysis comparing oral L-theanine with placebo in healthy and clinical groups reported no serious adverse events, and checked safety by comparing how many people dropped out of each group, and why (Gerolymos et al., 2026).
• Attention trials. In the single-dose attention studies, doses from 50 mg to about 500 mg were considered safe and well tolerated, with side effects no different from placebo (Cotter and Kilpatrick, 2026).
• Stress trials. A systematic review of nine randomized trials concluded that 200 to 400 mg a day may help with stress (Williams et al., 2020), and its authors reported that L-theanine was well tolerated with a lack of adverse events (Cotter and Kilpatrick, 2026).
• Sleep trials. Two reviews covering 22 randomized trials, at 50 to 1,000 mg a day for up to eight weeks, noted few and only mild side effects (Cotter and Kilpatrick, 2026).

If you notice anything unusual after taking L-theanine, stop and talk with your healthcare provider.

L-Theanine Side Effects and Your Heart

People often search for L-theanine and the heart. The human studies that measured heart rate and blood pressure looked at how the body responds to stress:

• Heart rate under stress. In a double-blind study of 12 people doing a mental arithmetic task, L-theanine reduced the rise in heart rate compared with placebo (Kimura et al., 2007).
• Blood pressure under stress. In a study of 14 people, L-theanine reduced the rise in blood pressure during mental tasks in the people whose blood pressure rose the most on placebo (Yoto et al., 2012).

Because L-theanine can affect blood pressure under stress, ask your clinician before using it if you take blood pressure medication.

Does L-Theanine Raise or Lower Your Heart Rate?

The studies that measured L-theanine and heart rate have not found it speeding up the heart. They looked at two situations:

• During stress. In the 12-person arithmetic study above, heart rate rose less after L-theanine than after placebo (Kimura et al., 2007).
• At rest. In a small pilot trial, 11 healthy men ate a sorbet containing L-theanine or a placebo sorbet, and heart rate, blood pressure and heart rate variability did not change significantly over the next 90 minutes in either group (Williams et al., Foods, 2020).

Many L-theanine drinks and capsules also contain caffeine, and too much caffeine can cause a fast heart rate (MedlinePlus: Caffeine). If your heart races after a combination product, check the caffeine amount on the label. If you notice a racing or irregular heartbeat after any supplement, stop it and talk with your healthcare provider.

L-Theanine and Heart Rate at a Glance

Study	Who	What it found
Mental arithmetic stress (Kimura et al., 2007)	12 people	Heart rate rose less after L-theanine than after placebo
Blood pressure under stress (Yoto et al., 2012)	14 people	Smaller rise in blood pressure during mental tasks in those whose pressure rose most on placebo
At rest (Williams et al., 2020)	11 healthy men	No significant change in heart rate, blood pressure or heart rate variability over 90 minutes

None of these studies found L-theanine speeding up the heart. Caffeine is the ingredient to watch in combination products; see L-theanine and caffeine and L-theanine benefits.

L-Theanine and Heart Rate by Product

Studies of L-theanine on its own have not found it speeding up the heart, so if a product with L-theanine leaves your heart racing, caffeine is the ingredient to check. What to know depends on what you take:

Product	What to know about heart rate
L-theanine on its own	In small studies, heart rate rose less during a stressful task than on placebo, and did not change significantly at rest over 90 minutes
L-theanine with caffeine (focus drinks and capsules)	Too much caffeine can cause a fast heart rate, so check the caffeine per serving and count coffee, tea and energy drinks in your daily total
Green or black tea	A 200 mL cup typically has about 8 to 25 mg of L-theanine, and green tea also contains caffeine (see green tea vs black tea)
Our Theanine Nasal Spray	36 mg of L-theanine per serving (one spray in each nostril), caffeine-free

If you notice a racing or irregular heartbeat after any supplement, stop it and talk with your healthcare provider. To keep your caffeine total in check, see how much caffeine is too much.

How Long Does L-Theanine Last?

L-theanine taken by mouth is absorbed quickly. Blood levels rise in step with the dose and peak about 45 to 50 minutes after taking it (Cotter and Kilpatrick, 2026). In a study of 12 healthy adults, the peak came about 0.8 hours after 100 mg, whether it was taken as a capsule or in green tea (Scheid et al., 2012).

The trials match that timing. A meta-analysis found that a single 200 mg dose taken 30 to 60 minutes before testing improved choice reaction time (Gerolymos et al., 2026), and a review of trials in healthy people measured effects in the first and second hour after taking it (Payne et al., 2025). Put simply, the studied effects of a single dose are measured over the first one to two hours.

How Long Does L-Theanine Stay in Your System?

The National Library of Medicine's drugs and breastfeeding database lists the half-life of theanine at about 1 hour, and notes that it should be mostly cleared from breast milk within 3 to 5 hours (LactMed: Theanine). In the body, L-theanine appears to be broken down into ethylamine and glutamic acid, which are passed in urine (Scheid et al., 2012).

Can You Take Too Much L-Theanine?

What was studied	Amount	How long
Attention trials	50 to about 500 mg, single doses	One dose
Stress trials	200 to 400 mg a day	Single doses up to eight weeks
Sleep trials	50 to 1,000 mg a day	Single doses up to eight weeks
Lab safety study in rats	Up to 4,000 mg per kg of body weight a day	13 weeks

Reviews place most of the studied doses in the 100 to 400 mg range (Cotter and Kilpatrick, 2026). In a 13-week animal safety study, the highest dose tested, 4,000 mg per kg of body weight a day, caused no consistent treatment-related adverse effects (Borzelleca et al., 2006). For comparison, a 200 mL cup of green or black tea typically has about 8 to 25 mg of L-theanine (Cotter and Kilpatrick, 2026). Stay within your label, and see our L-theanine dosage guide for how much to take and when.

Is L-Theanine Safe to Take Every Day?

Daily use has been studied. The stress trials ran from 17 days to eight weeks of daily dosing, and reviews describe 100 to 400 mg as safe and well tolerated over daily use (Cotter and Kilpatrick, 2026). In one four-week randomized trial, healthy adults took 200 mg a day (Hidese et al., 2019).

Does L-Theanine Make You Sleepy?

Reviews report that L-theanine does not appear to cause sedation or harm thinking, which suggests it may suit stressful moments at any time of day (Cotter and Kilpatrick, 2026). For evening use, our guide to L-theanine for sleep covers what the sleep trials found.

L-Theanine With Caffeine, Medicines and Blood Sugar

• Caffeine. L-theanine is often taken with caffeine, and a review of trials in healthy people found the pair could benefit attention and mood compared with placebo (Payne et al., 2025). Our L-theanine and caffeine guide covers ratios and timing.
• Blood sugar. In 39 healthy adults, 300 mg of L-theanine before a glucose drink did not significantly change the peak blood sugar rise; the authors called the effect extremely small and unlikely to raise clinical concerns (Yamaura et al., 2026).
• Medicines. If you take any regular medication, especially for blood pressure, sleep or mood, or sedatives, ask your clinician before adding L-theanine.

Green tea itself also contains EGCG and caffeine; see our guide to green tea extract and EGCG.

For what studies show L-theanine does, see our guide to L-theanine benefits. For side effects and study details in women, see our guide to L-theanine for women.

Who Should Check First

• Pregnancy and breastfeeding. LactMed reports no information on L-theanine supplements during breastfeeding and suggests it is probably best to avoid it while nursing a newborn or preterm infant (LactMed: Theanine). Talk with your healthcare provider first if you are pregnant or breastfeeding.
• Medication or a medical condition. Check with your clinician before use.

For a side by side with another calming option, see ashwagandha vs L-theanine, and for focus, L-theanine vs L-tyrosine. Our L-theanine ingredient page covers the basics. If you are also considering rhodiola, our guide to rhodiola vs L-theanine compares the two.

Where Our Theanine Nasal Spray Fits

Our Theanine Nasal Spray gives 36 mg of L-theanine per serving (one spray in each nostril), caffeine-free, with 30 servings per bottle, made with third-party tested ingredients. It is used in the nose instead of being swallowed like a capsule. Its dose is smaller than the 100 to 400 mg studied in most trials, so think of it as a convenient, on-the-go option that you can use as needed, alongside capsules if you want the studied amount. If you notice nasal irritation or anything unusual, stop use and talk with your healthcare provider. Our Melatonin Nasal Spray also lists 17 mg of L-theanine per 2-spray serving, in alkaline water. Questions are answered on the Theanine Nasal Spray FAQ, and more on the spray is in L-theanine nasal spray benefits and dosage. If you keep melatonin at home, see does melatonin expire for shelf life and storage.

Frequently Asked Questions

What are the side effects of L-theanine?

Across 31 randomized trials, no serious side effects were reported, and reviews describe side effects as few, mild and similar to placebo.

Is L-theanine safe to take every day?

Trials have used it daily for 17 days to eight weeks, and reviews describe 100 to 400 mg a day as safe and well tolerated.

Can you take too much L-theanine?

Human trials used 50 to 1,000 mg a day, mostly in the 100 to 400 mg range. Stay within your label.

Does L-theanine affect your heart?

In small studies, it eased the rise in heart rate and blood pressure during a stressful task. Ask your clinician first if you take blood pressure medication.

How long does L-theanine last?

Blood levels peak about 45 to 50 minutes after a dose, and trials measured its effects over the first one to two hours.

How long does L-theanine stay in your system?

Its half-life is about 1 hour, so most of a dose is cleared within a few hours.

Does L-theanine make you sleepy?

Reviews report that it does not appear to cause sedation.

Who should avoid L-theanine?

Check with a healthcare provider first if you are pregnant or breastfeeding, take medication, or have a medical condition.

Does L-theanine increase heart rate?

Studies have not found it speeding up the heart. In a small study of people doing a stressful arithmetic task, heart rate rose less after L-theanine than after placebo, and in a small pilot trial at rest, heart rate, blood pressure and heart rate variability did not change significantly over 90 minutes. If a product also contains caffeine, the caffeine can raise heart rate on its own.

Is L-theanine safe?

Across 31 randomized trials with 1,168 people, no serious side effects were reported, and reviews describe 100 to 400 mg a day as safe and well tolerated, with daily use studied for up to eight weeks. Check with a healthcare provider first if you are pregnant or breastfeeding, take regular medication, especially for blood pressure, sleep or mood, or have a medical condition.

Does L-theanine with caffeine affect heart rate?

Caffeine is the part to watch. Too much caffeine can cause a fast heart rate, while studies of L-theanine on its own have not found it speeding up the heart. Many focus drinks and capsules pair the two, so check how much caffeine each serving contains and count coffee, tea and energy drinks in your daily total. Our Theanine Nasal Spray is caffeine-free.

Can L-theanine cause heart palpitations?

The studies that measured heart rate did not find L-theanine speeding it up. If you feel palpitations after a product that also contains caffeine, check the caffeine amount, and stop and talk with your healthcare provider if it happens again.

Can I take L-theanine with blood pressure medication?

Ask your clinician first, because L-theanine reduced the rise in blood pressure under stress in one study.

Does L-theanine lower resting heart rate?

In 11 healthy men at rest, heart rate did not change significantly over 90 minutes. The calming effect on heart rate was seen during stress.

Is L-theanine bad for your heart?

The studies that measured heart rate and blood pressure have not found L-theanine speeding up the heart; in small studies, both rose less under stress than on placebo. Ask your clinician first if you take blood pressure medication or have a medical condition.

Does green tea raise your heart rate?

The L-theanine in tea has not been found to speed up the heart, but green tea also contains caffeine, and too much caffeine can cause a fast heart rate. A cup has about 8 to 25 mg of L-theanine, far less than the 100 to 400 mg used in most trials.

Does Theanine Nasal Spray contain caffeine?

No. Our Theanine Nasal Spray is caffeine-free and gives 36 mg of L-theanine per serving, one spray in each nostril. If you notice nasal irritation or anything unusual, stop use and talk with your healthcare provider.

Sources

• Gerolymos C, et al. Cognitive and affective effects of L-theanine: a systematic review and meta-analysis of 31 randomized trials. Mol Psychiatry. 2026. PubMed 42410082
• Cotter J, Kilpatrick HE. Effects of L-theanine on cognition, mood and sleep: evidence from human trials. Front Nutr. 2026. PubMed 42761839
• 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. PubMed 31758301
• Payne ER, et al. Effects of tea (Camellia sinensis) or its bioactive compounds L-theanine or L-theanine plus caffeine on cognition, sleep, and mood in healthy participants: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2025. PubMed 40314930
• Kimura K, et al. L-theanine reduces psychological and physiological stress responses. Biol Psychol. 2007. PubMed 16930802
• Yoto A, et al. Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses. J Physiol Anthropol. 2012. PubMed 23107346
• Scheid L, et al. Kinetics of L-theanine uptake and metabolism in healthy participants are comparable after ingestion of L-theanine via capsules and green tea. J Nutr. 2012. PubMed 23096008
• Drugs and Lactation Database (LactMed): Theanine. National Library of Medicine. PubMed 40460226
• Borzelleca JF, et al. A 13-week dietary toxicity and toxicokinetic study with L-theanine in rats. Food Chem Toxicol. 2006. PubMed 16759779
• 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. PubMed 31623400
• Yamaura S, et al. Clinical assessment of blood glucose responses to 300-mg dose of L-theanine, an amino acid unique to green tea, in a fixed-sequence, two-period trial. J Pharm Health Care Sci. 2026. PubMed 41845544
• Williams J, et al. The effect of L-theanine incorporated in a functional food product (mango sorbet) on physiological responses in healthy males: a pilot randomised controlled trial. Foods. 2020. PubMed 32210015
• MedlinePlus. Caffeine. National Library of Medicine. medlineplus.gov

These statements have not been evaluated by the Food and Drug Administration. This content is for education and is not medical advice. Our products are not intended to diagnose, treat, cure or prevent any disease. Talk with a healthcare provider before starting a new supplement if you are pregnant, breastfeeding or taking medication.