L-Theanine Side Effects: What the Trials Recorded, and How Long It Takes to Work

What are the side effects of L-theanine? In the trials, almost none. Studies at 200 mg to 400 mg a day have reported no significant adverse events, and the US Food and Drug Administration raised no questions about its safety as a food ingredient. The real cautions are narrower: extra drowsiness alongside sedatives, a possible effect on blood pressure, and too little data in pregnancy. Many of the “side effects” listed online actually belong to tea and caffeine.

L-theanine is an amino acid found mainly in tea leaves, sold as a supplement for calm and sleep. Because it is so often taken alongside caffeine, the two get blurred together, and that is where most of the confusion about its side effects comes from.

The short version

• Side effects in trials: very few. A four-week trial at 200 mg a day reported no adverse events, and Memorial Sloan Kettering’s integrative medicine service states that side effects have not been reported.
• Headaches, poor sleep, nausea and irritability are listed as effects of drinking too much tea, which contains caffeine, not of L-theanine itself.
• Real cautions: it may add to the drowsiness of sleeping pills and sedatives; it may blunt blood pressure rises under stress; and safety in pregnancy and breastfeeding is not established.
• How long it takes: blood levels peak about 50 minutes after a dose. Effects on stress and sleep scores were measured after four weeks.
• Dose in the research: usually 200 mg to 400 mg a day.

How this article was done

I have not taken L-theanine, and there is no test panel, because there was none. I read the main systematic review, the key human trials and how L-theanine is absorbed, the safety monograph from Memorial Sloan Kettering Cancer Center, and the FDA’s records on L-theanine as a food ingredient, including who funded each study. This is general information, not medical advice, and it follows my methodology.

What the trials actually recorded

The most useful safety data comes from trials that gave people L-theanine every day for weeks.

A 2019 randomised, placebo-controlled crossover trial in Nutrients gave 30 healthy adults 200 mg a day for four weeks. Nobody dropped out, and the authors reported that adverse events “were not apparent” and that there was no change in body weight or blood tests. The trial was funded by Taiyo Kagaku, which makes the L-theanine used, and two of its authors worked for the company, so it is not independent. But it is consistent with everything else I found.

A 2020 systematic review of nine randomised trials, from the University of Canberra, concluded that 200 mg to 400 mg a day “may assist in the reduction of stress and anxiety in people exposed to stressful conditions”, while calling for longer and larger trials.

Memorial Sloan Kettering’s patient guide is blunter still: “Side effects have not been reported.” It also notes that L-theanine does not affect the CYP450 liver enzymes that drive many drug interactions.

On the regulatory side, the FDA has twice responded with “no questions” to notices that L-theanine is generally recognised as safe as a food ingredient, in 2007 and in 2014. That is not approval as a medicine, and it was assessed for food use rather than high-dose supplements, but it is a meaningful safety signal.

The side effects that are really tea and caffeine

Many lists of “L-theanine side effects” include headaches, trouble sleeping, nausea, irritability and stomach pain. Memorial Sloan Kettering lists those too, but under a different heading: as effects of drinking too much tea. Tea contains caffeine, and those are classic caffeine effects.

The confusion is understandable. L-theanine is often sold in combination with caffeine, in “focus” capsules and energy drinks, and people take it alongside coffee. If you feel jittery, wired or unable to sleep after a product that contains both, the caffeine is the far more likely cause. Check the label: if caffeine is listed, the L-theanine is probably not the problem.

The real cautions

Sedatives and sleeping pills

Memorial Sloan Kettering advises talking to your doctor if you take sleep medicines such as lorazepam, diazepam or zolpidem, because the combination may increase drowsiness. In animal studies, L-theanine added to the effect of the sedative midazolam; whether that matters in people is not known. If you take any sedative, ask before adding it.

Blood pressure

L-theanine may lower the rise in blood pressure that comes with stress. In a small 2012 crossover trial of 14 adults, 200 mg taken before mental tasks reduced the blood-pressure increase in people whose pressure normally rose the most, and lowered their tension-anxiety scores. For most people that is a benefit, not a side effect. If you take blood-pressure medicine or tend to have low blood pressure, it is worth checking with your doctor first.

Pregnancy, breastfeeding and children

There is not enough research to say L-theanine is safe in pregnancy or breastfeeding, and Memorial Sloan Kettering advises talking to a healthcare provider first. The same applies to children.

Does L-theanine affect your heart, or cause anxiety?

I found no trial reporting heart problems from L-theanine. The heart-related finding in the research runs the other way: a smaller rise in blood pressure under stress. If you notice palpitations after a product containing L-theanine, check whether it also contains caffeine, and speak to a doctor if they persist or come with chest pain, dizziness or breathlessness.

Likewise, I found no trial in which L-theanine made anxiety worse; the trials measured reductions. Individual reactions are always possible, and if a supplement makes you feel worse, stop taking it.

How long does L-theanine take to work?

It depends on what you are looking for.

• A calming effect from a single dose: a study of how L-theanine is absorbed found blood levels peaked about 50 minutes after taking it, and fell with a half-life of roughly an hour. Single-dose studies typically measure effects within this window.
• Changes in stress and sleep scores: the 2019 trial measured them after four weeks of daily use.

So if you take it for a stressful event, take it 30 to 60 minutes beforehand. If you want to know whether it helps your everyday stress or sleep, give it a few weeks at a steady dose. What the anxiety trials found, including the one in diagnosed anxiety disorder, is in L-theanine for anxiety.

How much is too much?

Most trials used 200 mg to 400 mg a day, and those are the doses with a safety record behind them. There is no official upper limit for L-theanine. There is also no evidence that taking more than the studied doses works better, so there is little reason to exceed them.

Who should be careful

• People taking sedatives, sleeping pills or anti-anxiety medicines.
• People taking blood-pressure medicine or prone to low blood pressure.
• Anyone pregnant or breastfeeding, and children.
• Anyone whose product also contains caffeine and who is sensitive to it.

If you are comparing L-theanine with other options for everyday stress, the evidence for magnesium is in magnesium for anxiety, and the supplements with cortisol data are compared in the best supplement to lower cortisol. For persistent or severe anxiety, talking therapies and medical care have far stronger evidence than any supplement.

Comparing options? All seven of the main stress supplements are ranked on their trial evidence in best supplements for stress.

Choosing a product? Five popular L-theanine supplements are compared on form (Suntheanine or generic), dose, verified testing and cost in the best L-theanine supplement.

Common questions

What are the side effects of L-theanine?

In trials at 200 mg to 400 mg a day, side effects were rare or not reported. The main cautions are extra drowsiness with sedatives, a possible blood-pressure-lowering effect, and unknown safety in pregnancy and breastfeeding.

Does L-theanine have side effects on the heart?

No trial I found reported heart side effects. In one small study it reduced the rise in blood pressure under stress. Palpitations after an L-theanine product are more likely to come from caffeine in the same product.

Can L-theanine cause anxiety?

The trials measured reductions in stress and anxiety, not increases. If you feel more anxious after a product, check it for caffeine and stop taking it.

How long does it take for L-theanine to work?

Blood levels peak about 50 minutes after a dose. Effects on everyday stress and sleep were measured after about four weeks of daily use.

Is it safe to take L-theanine every day?

Trials of daily use for four weeks reported no adverse events, and it has a long history in tea. Long-term trials are limited, so check with a doctor if you plan to take it for months, especially alongside medication.

Can I take L-theanine with coffee?

Yes. Many studies have tested the two together, and it is a common combination. If you are sensitive to caffeine, the caffeine is the part to watch.

Sources

• 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 — the four-week trial, its safety findings and its funding.
• 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 — the nine-trial review and the 200–400 mg range.
• Memorial Sloan Kettering Cancer Center — L-Theanine — side effects, sedative interactions and pregnancy advice.
• US FDA — GRAS Notice No. 209, L-theanine — the 2007 “no questions” response; a second notice, No. 501, followed in 2014.
• 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 — the blood-pressure trial.
• van der Pijl PC et al. Human disposition of L-theanine in tea or aqueous solution. Journal of Functional Foods. 2010 — the 50-minute peak and one-hour half-life.