L-Theanine: Dosage, Benefits, and Side Effects

Key Takeaways

• L-theanine is an amino acid found mainly in green and black tea. Research suggests that around 200 to 400 milligrams (mg) per day may help manage anxiety and stress, though more research is necessary.
• L-theanine may also help improve sleep quality, boost alertness, support immune function, and manage stress-related high blood pressure. However, the evidence for these benefits is still preliminary.
• L-theanine is generally safe to consume in tea or supplements, but people who are pregnant, nursing, or receiving cancer treatment should talk with a healthcare professional before use. Some compounds in tea can reduce the effectiveness of certain chemotherapy medications.

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

It may offer health benefits such as managing anxiety and stress. It may also help reduce sleeping problems such as insomnia.

However, L-theanine can also cause side effects, and it may not be suitable for everybody. It’s best to speak with a healthcare professional if you are considering taking L-theanine.

Read on to learn more about the possible benefits of L-theanine, potential side effects, and dosage information.

L-theanine has numerous possible health benefits.

Anxiety and stress relief

A 2019 review found that around 200 to 400 milligrams (mg) of L-theanine per day could help to ease anxiety and stress.

Some effects of L-theanine may occur within a few hours after taking it. There may also be long term benefits, but more research is necessary.

Increased alertness

L-theanine occurs naturally in tea. Some 2025 research suggests that drinking L-theanine in tea may help contribute to the feeling of alertness.

The research also suggests that catechin, an antioxidant compound also present in tea, can contribute to alertness.

More research is necessary to determine the role L-theanine plays in making people feel more alert and awake when consuming it in tea.

Better immunity

Some research suggests that L-theanine may improve the function of the body’s immune system. A 2016 study found that L-theanine could help decrease upper respiratory tract infections.

According to a 2023 review, L-theanine may help reduce immunosuppression caused by intense exercise. It may also be able to help prevent colds and flu by strengthening the immune system.

Cancer prevention and treatment

While more research is necessary, some studies suggest that L-theanine may help prevent or manage cancer.

For example, a 2021 study explains that L-theanine can encourage the death of cancer cells and help prevent them from growing. However, it is unclear whether it can help prevent metastasis, or the spread of cancer to other parts of the body.

The study found that L-theanine can reduce the movement of prostate cancer cells. More research is necessary to study the possible benefits of L-theanine in cases of prostate cancer and other types of cancer.

A 2023 review notes that cellular and animal studies have shown the possibility of L-theanine in supporting the immune system when fighting against tumors.

Further research is necessary to explore the role of L-theanine in cancer prevention or treatment.

Blood pressure management

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

While more research is necessary, the ability of L-theanine to manage or reduce stress can help manage stress-related high blood pressure (hypertension).

Improved sleep quality

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

It is unclear exactly how much L-theanine can help with sleep. It is best to speak with a healthcare professional for information about the quantity that may help and how soon before bed it is best to take it.

Generally, it’s safe to drink L-theanine in teas and take supplements that contain L-theanine within a recommended dose.

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

However, it’s important to note that, while some research shows promising results for L-theanine’s anti-tumor properties, teas containing amino acids may also include other ingredients that could be harmful to individuals undergoing cancer treatment.

According to the Memorial Sloan Kettering Cancer Center, the polyphenol EGCG found in green tea can reduce the efficacy of some chemotherapy drugs.

For that reason, it’s crucial for those taking chemotherapy drugs to talk with a healthcare professional before drinking tea.

Because of the caffeine content in tea, consuming large amounts of tea can lead to:

• nausea
• upset stomach
• irritability

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

It’s best to ask a healthcare professional about what’s safe for you.

Because there’s been no conclusive research, a safe L-theanine dosage recommendation isn’t clear. 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 professional for guidance on dosage.

L-theanine is an amino acid that you can primarily find in green and black tea, as well as in some mushrooms. It may offer numerous health benefits, such as reducing anxiety, relieving stress, and helping with sleep.

L-theanine is generally safe to consume. However, it is best to seek medical advice if you are pregnant, nursing, or receiving cancer treatment. Some amino acids can impact the effectiveness of treatments such as chemotherapy.

Speak with a healthcare professional for information on whether they recommend L-theanine based on your individual circumstances, as well as how much may be safe for you to take.