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Mental health in the United States is at an all-time high. As a result, 41% of American adults have experienced psychological distress since 2020.

L-theanine is an increasingly popular supplement used for a variety of purposes. This is due to the noted benefits it provides to the body as a whole. It does this as a relaxant and focus booster that helps the body feel at its best. But is it safe to take, and are there any downsides?

This article will address the potential L-theanine side effects, if any. Read on to see what you can expect when taking theanine supplements.

What Is L-Theanine?

“What is L-theanine?” we hear you asking. L-theanine is an amino acid that occurs naturally in tea and certain mushrooms. It can also be called γ–glutamylethylamide, which is another scientific name for it. There are two forms of Theanine: D-Theanine and L-theanine.

L-theanine is used as a relaxing agent, usually drunk as a beverage. This relaxation boosts brain dopamine, serotonin, and GABA levels. Research so far has declared that L-theanine is safe to consume. However, it’s important to note that there isn’t enough information to know if it’s safe for pregnant or breastfeeding women.

In children aged 8-12, L-theanine consumption is considered safe in small doses and not long-term use. L-theanine has been safely used for up to six weeks. This is in a daily oral L-theanine dosage of 200-400mg, the same for adults.

Potential L-Theanine Benefits

Research is still being done on the benefits of medical products like L-theanine. However, there is an interest in its use for other purposes. There is one study that suggests L-theanine can help with the following:

  • Depression (-)
  • Anxiety (-)
  • Sleep (+)
  • Cognitive function (+)

Because L-theanine is a relaxing agent, it’s been shown to potentially reduce the effects of depression and anxiety and improve sleep quality and cognitive functions such as verbal fluency.

What Are the Side Effects of L-Theanine?

While low doses of L-theanine are considered safe, several potential side effects exist. A common question asked is, “What are the side effects of L-theanine?”


Because L-theanine has a soothing effect when consumed, the most common side effect users report is drowsiness. Some consider this a negative side effect, and it may impair abilities such as driving.


Nausea can be a side-effect of L-theanine but is typically only experienced if it’s consumed on an empty stomach. For this reason, taking L-theanine with food is recommended.


Another side effect that has been reported is dizziness. When this happens, it’s typically because the dosage is too high. Reduce the dose, and the dizziness should stop.


Some people report headaches for a couple of days after consuming medical products like L-theanine. If your headache continues for a week or more, consult your medical provider.

How Long Does L-Theanine Last in the Body?

Another common question about L-theanine is, “How long does L-theanine last in the body?” You should feel the effects of L-theanine within 30 – 60 minutes after consumption. The results can last up to 8 hours.

Understanding L-Theanine Side Effects

While research is ongoing, a study has suggested several L-theanine benefits. This supplement is considered safe if you follow your doctor’s instructions regarding L-theanine dosage, etc. If you feel any unwanted L-theanine side effects listed, cease taking it and consult your doctor.

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