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L-theanine is an amino acid found primarily in green and black tea and some mushrooms. It’s also available in pill or tablet form.

It’s said to help ease anxiety, stress, and reduce insomnia.
Before trying it out yourself, learn more about the potential health benefits, as well as any possible risks or complications.

Most known for helping people relax, L-theanine has other potential health benefits, including:

Anxiety and stress relief

Sipping on a hot cup of tea can help you feel at ease, and research suggests that it not only relaxes the mind, but it also does so without causing drowsiness (1).

In a review of five randomized controlled trials that included 104 participants, four trials linked L-theanine with reduced stress and anxiety in people experiencing stressful situations (2).

Another study published in the Journal of Clinical Psychiatry focused on people living with schizophrenia or schizoaffective disorder. Researchers found that L-theanine decreased anxiety and improved symptoms (3).

Increased focus

Paired with caffeine, L-theanine may help increase focus and attention.

A small study found that a combination of L-theanine (97 milligrams, or mg) and caffeine (40 mg) helped a group of young adults focus better during demanding tasks. A typical cup of coffee contains 95 mg of caffeine (4, 5).

The study’s participants also felt more alert and less tired in general.

Better immunity

Some research suggests that L-theanine may improve the function of the body’s immune system. One study published in the journal Beverages found that L-theanine could help decrease upper respiratory tract infections (6).

Another one found that green tea catechins— antioxidants— and theanine could be effective at preventing the flu (7, 8).

Another study found that L-theanine could help improve inflammation in the intestinal tract. However, more research is needed to confirm and expand on these findings (6, 9).

Tumor and cancer treatment

L-theanine has also been associated with amplifying the anti-tumor effects of certain chemotherapy drugs. Because of these promising findings, researchers expect that L-theanine could also help improve chemotherapy’s ability to fight cancer (10).

Although there’s no definitive evidence to show that tea prevents cancer, a number of studies suggest that people who regularly drink tea have lower rates of cancer (10, 11, 12).

Researchers of one study in China found that women diagnosed with ovarian cancer who drank at least one cup of green tea a day lived longer than those who didn’t (13).

Another study that looked at tea drinkers compared to nondrinkers found that women who drank green tea were 32 percent less likely to develop pancreatic cancer (14).

Blood pressure management

L-theanine may be beneficial for those who experience increased blood pressure in stressful situations.

One study found that people who usually experienced higher blood pressure after specific mental tasks found that L-theanine helped reduce an increase in blood pressure.

In the same study, the researchers noted that caffeine had a similar but less beneficial effect (15).

Some research indicates that L-theanine could be beneficial for a good night’s sleep, which could be because it helps to promote relaxation.

Researchers in one study found that doses of 250 mg and 400 mg of L-theanine greatly improved sleep in animals and humans (16).

Also, 200 mg of L-theanine was shown to help reduce resting heart rate, pointing to its ability to promote relaxation (16).

L-theanine may also help boys diagnosed with attention deficit hyperactivity disorder (ADHD) sleep better.

A double-blind study looked at the effects of L-theanine on 98 boys ages 8 to 12 years old. A randomized group was given two 100 mg chewable tablets of L-theanine twice daily. The other group received placebo pills.

After 6 weeks, the group taking L-theanine had longer, more restful sleep. While the results are promising, more research is needed before it can be proven safe and effective, especially for children (17).

Other research suggests that L-theanine improved sleep quality for those diagnosed with schizophrenia (18).

Shop for L-theanine online.

There are no confirmed or direct side effects of consuming L-theanine. Generally speaking, it’s safe to drink teas and take supplements that contain L-theanine.

But it’s important to note that the Food and Drug Administration (FDA) doesn’t regulate supplement. Manufacturers of those products bear the weight of the responsibility to make sure their products are safe (19).

However, though some research shows promising results for L-theanine’s anti-tumor properties, teas that contain amino acids can have other ingredients that could be harmful to people being treated for cancer.

According to Memorial Sloan Kettering Cancer Center, the polyphenol EGCG found in green tea can reduce some chemotherapy drugs’ efficacy, such as bortezomib (20, 21).

For that reason, it’s crucial for those taking chemotherapy drugs to talk with their healthcare provider before drinking green tea as part of their treatment plan.

While there haven’t been reported side effects from taking L-theanine, because of the caffeine content, consuming large amounts of green tea can lead to:

  • nausea
  • upset stomach
  • irritability

Women who are pregnant or breastfeeding should also limit how much tea they drink to avoid over-caffeinating.

It’s best to ask your healthcare provider about what’s safe for you. The same advice applies to children as well.

Because there’s been no conclusive research, a safe L-theanine dosage recommendation isn’t known. But following general caffeine consumption guidelines can be helpful if you’re drinking tea.

It’s best for those taking an L-theanine supplement to consult a medical provider for guidance on dosage.