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L-Theanine is an amino acid (also known as a ‘building block of protein’)  and is thought to be the source of the flavor ‘umami’, a savory taste that is hard to define. L-Theanine is derived from tea leaves from a plant that grows to over nine meters in height and has dark green leaves with serrated edges. The amino acid can also be found in a specific kind of mushroom, Boletus badius, that is found across the United States in the summer and fall months. The mushroom can grow from 4-12 cm tall and is a reddish brown color, with white flesh. It is thought to have a relaxing effect on the body and help the immune system.

The Bottom Line – L-Theanine is an amino acid and may be beneficial in inducing a state of relaxation; it may also have immunologic effects.

L-Theanine Health Benefits

L-Theanine can be used to treat anxiety due to its relaxing properties and may also be beneficial in preventing certain degenerative brain conditions.

Below is a list of the possible benefits we’ve found: 

L-Theanine Can Help With Reducing Anxiety Levels

L-Theanine was used in a Japanese study where college students took doses of L-Theanine or a placebo alternative. The college students who took the L-Theanine supplement reported reduced anxiety levels and also had smaller increases in blood pressure while under stress (physical or mental) than those students who took the placebo. Another Japanese study with students who undertook stressful work in a pharmacy showed that those who took L-Theanine had lower stress levels than those who didn’t. [1][2]

L-Theanine Can Help Improve Sleep Quality

One study based in Canada found that taking an L-Theanine supplement improved several aspects of sleep quality in boys under the age of 18 diagnosed with Attention Deficit Hyperactivity Disorder (ADHD). It is thought that L-Theanine’s positive effects on sleep are due to its reductive impact on the stress hormones cortisol and epinephrine. [3]

L-Theanine Can Help With Psychotic Problems

One study based in Israel found that participants aged 19 to 55 suffering from schizophrenia or schizoaffective disorder reported a reduction in anxiety when taking L-Theanine supplements. They also found that the effects of antipsychotic treatment were increased compared to participants who did not take L-Theanine. [4]

L-Theanine Can Help With Depression

In 2017 the first study of the effects of L-Theanine on patients with depression was carried out. Men and women with depression took a 250 mg supplement of L-Theanine as well as their usual medication, for eight weeks. The patients reported multiple benefits including fewer sleep disturbances, improved mood and improved cognitive functioning. [5]

The Bottom Line – L-Theanine seems to have a positive effect on the body’s neurotransmitters, leading to improvement in mood and a reduction in anxiety levels.

How To Use L-Theanine

The methods of using L-Theanine vary from tea to tablets.

The most common methods of consuming L-Theanine are:

  • As a drink (in tea leaves or tea powder)
  • As a tablet or capsule (to treat various mental conditions)

The optimal dosage amount for L-Theanine is not yet agreed upon by researchers. The general guidelines for adult men is 100-200 mg/day for men and women; up to 250 mg/day has been used safely in research studies.

Are There Any Foods That Contain L-Theanine?

The only food or drink products that contain L-Theanine are one species of mushroom, or tea; black tea contains L-Theanine but green tea is thought to be a richer source.

L-Theanine Side Effects List

There are always risks to taking any kind of drug or supplement even if they have been used safely by others so talk to your doctor before changing your diet to include more or less L-Theanine than you are used to.

The following side effects could potentially be:

  • Appetite loss
  • Low blood pressure
  • Difficulty concentrating
  • Diarrhoea
  • Dizziness
  • Stomach discomfort
  • Headaches
  • Nausea 

Note: Contact your doctor before stopping or starting a supplement to discuss whether it is appropriate for your individual situation.

Is L-Theanine Safe?

Consumed at the daily recommended doses, L-Theanine has been found to be safe. However when used at higher doses there are some risks. Some of the risks include a sedative effect and in interference with the ability to concentrate properly

When Should I Not Take L-Theanine?

There are no known demographics for whom L-Theanine is deemed to be dangerous when taken sensibly. If you are pregnant or breastfeeding we would recommend you consult with your doctor prior to supplementing this amino acid.

Has L-Theanine Been Linked To Any Deaths?

L-Theanine has not been linked to any known deaths.

The Bottom Line – L-Theanine has several positive properties and is generally safe to consume within daily recommended limits. It can in large quantities cause over-relaxation and a sedative effect.

Where Can I Buy L-Theanine & Do I Need A Prescription?

You can purchase tea containing L-Theanine at most food stores; you can purchase supplements online and a prescription is not needed. Fresh mushrooms containing L-Theanine can be sourced in the USA during the summer and fall months, or acquired in dried form year-round.


L-Theanine is an amino acid that has several positive psychoactive properties and can reduce anxiety and sleep disturbances. Sensible consumption within recommended dosage limits is considered safe for most people; contact your doctor if you have concerns about the effects of L-Theanine or its inclusion in your diet.

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Fact Checked

This article has been reviewed and fact-checked by a certified nutritionist, and only uses information from credible academic sources.