April 22, 2022 5 min read

L-theanine is an amino acid that's not naturally found in the body, but it still may do important work when taken in supplement form. Some evidence suggests that it works similarly to glutamate, an amino acid that's responsible for many important neural processes.

Some evidence suggests that L-theanine can help regulate neurochemicals like GABA, dopamine, and serotonin. For that reason, many people believe it to have various benefits, like improving sleep or helping to manage mood disorders. If you're familiar with the potential benefits of L-theanine, you've probably come to an important question–are there any L-theanine side effects to be aware of?

Fortunately, L-theanine is thought to be safe for most people. Product quality is the main concern regarding the safety and side effects of L-theanine supplements. Before you get started with your L-theanine dosing routine, here are the potential adverse effects you need to know:

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Table of Contents
Is L-Theanine Safe?
What are the Side Effects of L-Theanine?
L-Theanine Drug Interactions
Is L-Theanine Addictive?
Who Shouldn’t Take L-Theanine?
L-Theanine Side Effect FAQs

Key Takeaways

  • L-Theanine is naturally occurring in most teas and is thought to be safe for daily use.
  • There are no known side effects related to L-theanine specifically, but some supplements may contain other ingredients, like caffeine, that have their own side effects profile.
  • L-Theanine should not be taken with some medications, including some blood pressure medications and cancer treatments.
  • You should talk to your doctor before taking L-Theanine and be sure to choose a high-quality supplement.

A cup of green tea, a rich natural source of L-theanine.

Is L-Theanine Safe?

Generally, L-theanine is thought to be safe for most people when a high-quality supplement is used responsibly. L-Theanine is an amino acid that's naturally occurring in tea and other herbal products, and its safety profile is similar to that of a cup of green tea. Of course, there are some drug interactions and quality precautions to be aware of. Let's dig into the possible side effects:

What are the Side Effects of L-Theanine?

Based on anecdotal reports and various trials, we know that there are no known side effects related exclusively to L-Theanine supplementation. Often, side effects are linked to poor product quality or adverse drug interactions. Many products also contain other herbal additives and supplements, like GABA or caffeine, which could have side effects of their own. Although rare, some of the most commonly reported side effects include:

Allergic Reaction

L-Theanine is not known to pose any severe allergy threats, but some people have had allergic reactions to other components of L-theanine supplements. Keep in mind that all supplements are made differently and it's crucial that you find one that's made with ingredients that work for you.

In any case, you should get emergency medical help if you have signs of an allergic reaction, including:

  • Hives
  • Breathing difficulty
  • Swelling of the lips, tongue, face, or throat


Many L-theanine supplements contain caffeine. Some people who are sensitive to caffeine may experience increased energy levels to the point of feeling jittery or anxious. This isn't linked directly to L-Theanine consumption, but it is a possible side effect to be aware of.


Again, caffeine in some L-theanine supplements may cause a headache for some people. Alternatively, some people may experience a headache if they quit using an L-theanine supplement containing caffeine cold turkey.

Nausea or Upset Stomach

If your L-theanine supplement contains caffeine, it may also have side effects like nausea or upset stomach when taking in high doses.

Interactions With Other Supplements or Medications

L-theanine could interact with certain prescription and over the counter medications, so it's important to discuss supplementation with your doctor before you begin. We'll discuss this more below.

A person holding a dose of L-theanine, which in large doses can interact with some prescription medications.

L-Theanine Drug Interactions

L-Theanine is not known to interact with many medications, but it’s full drug interaction panel has not been tested. For now, we know that L-theanine, especially in large doses, may interact with the following medications:

  • Anithypertensive drugs (Drugs taken to lower blood pressure)
  • Certain anti-depressants
  • Certain cancer medications, such as bortezomib

You should also pay attention to the caffeine content of L-theanine supplements and factor it into your daily caffeine intake if taking other supplements or food products containing caffeine.

Is L-Theanine Addictive?

L-Theanine is not known to have any habit forming properties. However, many L-theanine supplements also contain caffeine, which can be habit forming. It's important to understand the risks and benefits of the full ingredients profile of the L-theanine supplement you choose.

Who Should Not Take L-Theanine?

There are little to no risks associated with taking L-theanine, assuming the supplements you choose are well-made and safe. Some L-theanine supplements contain caffeine, so people may have a sensitivity to caffeine and may need to pay attention to the caffeine dosage in certain formulas. There are a few rare cases where L-theanine supplementation could be dangerous, including:

  • When taking certain cancer medications, such as bortezomib, where L-theanine may decrease the drug’s efficacy.
  • When experiencing nausea, upset stomach, or irritability where caffeine may make it worse. In this case, an L-theanine supplement without caffeine may still be used.

In any case, always talk to your doctor before making changes to your wellness routine. Keep in mind that L-theanine is sold as a supplement in the U.S., and supplements are not as well-regulated as pharmaceuticals and over the counter medications. You should make sure that you only buy supplements from a trustworthy source and ensure that you are using the correct L-Theanine dosage to reduce the chance of side effects.

L-Theanine Side Effects FAQ

Can you take L-Theanine everyday?

Yes, L-theanine is typically used daily in one or more daily doses. Consistency is the best way to achieve optimal benefits from your L-Theanine dosing routine

Is it OK to take L-theanine long term?

Research has found that using L-theanine over an 8-week period is safe. Other experts suggest that daily L-theanine use over a longer period of time is likely as safe as taking green tea daily. There’s no research to identify the benefits or drawbacks of taking L-theanine for extended periods of time.

Can theanine worsen anxiety?

Some evidence suggests that L-theanine can possible increase serotonin levels to help manage anxiety, but it’s also ben found to possibly reduce serotonin levels in some people, which may have the opposite effect. This needs to be evaluated on a case by case basis, so work with your doctor if you intend to use L-theanine for anxiety or if you believe that taking L-theanine caused your anxiety to worsen.

Does L-theanine raise blood pressure?

No, research has found that L-theanine may actually help to decrease blood pressure, but more research is needed to confirm these effects. However, if the L-theanine supplement contains caffeine, be aware that caffeine can increase blood pressure.

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