L-Theanine Side Effects: Top 10 Potential Adverse Effects  

"L-Theanine side effects are not often considered when people look into this powerful supplement. Some users do report experiencing a variety of mild side effects from taking an L-Theanine supplement. In this article, we’ll explore the top 10 Theanine side effects that you should know about!   Theanine is considered to be one of the top natural calming supplements available today. It was traditionally extracted from the leaves of the green tea plant, along with caffeine, polyphenols, and catechins. Recent studies have indicated that L-Theanine benefits include the potential to produce relaxation without causing sedative effects. [caption id=""attachment_2001"" align=""alignright"" width=""680""] Green tea is the traditional source of L-Theanine and has been used in Asia for thousands of years[/caption]

L-Theanine Benefits and Effects

L-Theanine supplements have a great list of benefits and effects. Some of the top L-Theanine benefits include: promoting relaxation[1], reducing stress levels[2], helping improve sleep quality[3], and promoting a healthy state of mind[4]. L-Theanine benefits have been known across the world and especially in eastern Asia for thousands of years. Although the ancient civilizations may have had a different name for it, they too understood the relaxing and calming effects of this great compound. In fact, if it hadn’t been for the long history of use of green tea in Asia and its subsequent export to the West, none of us would know of the great L-Theanine benefits we can experience today! For more information about L-Theanine’s great benefits and how you can use it to improve your overall health, check out our blog post on the Top 6 L-Theanine Benefits! You may also be interested in checking out our informative product description.  

5 Important L-Theanine Side Effects Factors:

Before going into the potential adverse effects from taking Theanine, we’ll outline five very important factors that can influence how you experience this supplement. The five factors below are important deciders of whether or not you will experience any L-Theanine side effects.

1. Dosage

The L-Theanine dosage that you use is a very important factor in deciding whether you may or may not feel any L-Theanine side effects. The recommended L-Theanine dosage is around 100-200 mg, once or twice daily. However, because L-Theanine is such a safe dietary supplement, many people use a higher dosage to experience stronger Theanine effects. Using a higher dosage may increase your chances of experiencing negative Theanine side effects.  

2. Interactions

In this case, ‘interactions’ refers to the interactions between L-Theanine and other substances or medications. The potential interactions of L-Theanine have not yet been sufficiently studied, so it is not recommended to use L-Theanine with medication for safety reasons. Potential interactions include L-Theanine with stimulants like caffeine (positive interaction) or ephedrine, and with blood-pressure-lowering medication.[5] Other L-Theanine interactions may include melatonin with L-Theanine and L-Theanine with alcohol.[6]  

3. Usage

[caption id=""attachment_1991"" align=""alignright"" width=""298""] Liftmode's L-Theanine, 99% Purity 5 Grams[/caption] We’ve explored how your L-Theanine dosage is an important factor in whether or not you’ll experience L-Theanine side effects. However, what about ‘usage’? By this, we mean how regularly you use L-Theanine, as well as when and how you take it. Using L-Theanine regularly may have the potential to result in your body developing a tolerance to its effects. This could mean that even if you feel some negative effects in the beginning, after using L-Theanine for a while you might not experience side effects anymore. However, it could also mean that you’ll only start to experience L-Theanine side effects after using it for a long time. Cycling L-Theanine (not using it every day) is a good way to prevent feeling this from occurring. Another important factor is the time of day that you take L-Theanine. Because it is absorbed in your stomach, you can increase the potency of its effects by taking an L-Theanine supplement on an empty stomach. However, this could potentially result in stomach cramps or nausea.  

4. Purity

The purity and quality of the L-Theanine supplement that you take are very important factors when it comes to thinking about potential side effects. Many Theanine Reddit posts talk about potential purity issues when it comes to L-Theanine. There are many vendors selling this supplements, but not all vendors are equal. The best thing is to try to find the purest and highest quality L-Theanine available. An important note to make is whether the supplement is pure “L-Theanine” or contains some of the enantiomer “D-Theanine”. For the health benefits, you’ll really just want pure L-Theanine. Make sure that the supplement manufacturer does not use fillers or excipients, and attaches a third party Certificate of Analysis (CoA) to confirm the purity rating of their product. Another good method is to check online and read through L-Theanine reviews where people rate the company that you are buying from. People will post their experiences with the product, and let you know if they experienced any L-Theanine side effects.  

Individual metabolism

Everybody is different. Our bodies all respond in different ways to various substances and supplements. While one person might find that they only need 50mg of L-Theanine for relaxation, somebody else might have a different metabolism and may need over 250mg to feel any real L-Theanine benefits. These differences in individuals are also related to potential L-Theanine side effects. For example, one person may simply have more of an affinity for Theanine side effects than another. This variation is often due to genetic factors. It can also be caused by different lifestyle and dietary choices, as well as your psychological state.

Top 10 Potential L-Theanine Side Effects:

The following is a list of ten possible side effects from using L-Theanine. It’s important to remember that L-Theanine is considered to be a very safe dietary supplement and is regarded by most nutritionists as being free from adverse effects. Nevertheless, some people do still experience some L-Theanine side effects, mostly due to the factors we outlined above.  

Appetite Loss

Despite the fairly large amount of information on the internet about Theanine and appetite, there are actually no scientific articles available to date confirming that L-Theanine can affect your appetite. However, there is a fair amount of anecdotal evidence that this supplement may reduce your appetite, as seen on the L-Theanine Erowid and Theanine Reddit post forums.  

Lower blood pressure

There is a solid amount of research indicating that L-Theanine may have the potential to temporarily reduce your blood pressure. For many people, this would be considered as an L-Theanine benefit. However, if you suffer from low blood pressure, you may want to consider using a lower-than-normal Theanine dosage.[7]  

Difficulty concentrating

Again, this is a controversial L-Theanine side effect. Many studies have indicated that Theanine is a great supplement to improve cognitive performance, especially when combined with caffeine.[8] However, some people do report feeling a Theanine ‘brain-fog’, especially when using higher doses. This is written about by some people on Theanine Reddit posts. This side effect would be considered as being a direct result of individual variation in people’s metabolism and bodies. Most people don’t experience this, but a few do. To avoid this effect, we’d recommend using an L-Theanine caffeine combination.  


This is another anecdotal side effect. No studies to date have confirmed that diarrhea is a potential L-Theanine side effect. In fact, at least one study has shown that Theanine may have the potential to improve gastrointestinal discomfort when suffering from opioid withdrawal.[9] There is a Theanine Reddit post about this side effect felt by some. However, people who experience this side effect tend to be combining L-Theanine and caffeine. Caffeine is known to act as a laxative and can cause diarrhea.[10] So, it’s difficult to tell whether this is really a potential L-Theanine side effect or if it is caused by caffeine.  


Dizziness appears to be one of the most commonly reported adverse effects for Theanine. There are no studies to-date solidly confirming that L-Theanine alone can cause dizziness. One study, on eHealthMe, reported that 1 out of 35 persons experienced dizziness when using an L-Theanine supplement.   [caption id=""attachment_2002"" align=""alignright"" width=""680""] There is strong evidence that L-Theanine improves relaxation and calmness[/caption]

Gastrointestinal Discomfort

Some people report feeling slight gastrointestinal discomfort when using L-Theanine supplements. This is quite a rare side effect and is almost always mild in nature. Using a lower dose should resolve this issue. Also, if you experience gastrointestinal discomfort when taking L-Theanine on an empty stomach, try using L-Theanine an hour or two after eating instead. This may help to reduce the L-Theanine negative side effects.  


One of the top reported L-Theanine negative side effects is the experience of headaches. There is a fair amount of clinical research confirming that this may be one of the few genuine Theanine side effects. For example, a 2008 study found that people taking L-Theanine supplements had increased ‘headache’ ratings compared to placebo. However, the combination of L-Theanine and caffeine reduced the headaches.[11]  To help prevent experiencing headaches when taking an L-Theanine supplement, we recommend using the minimum active dose (the lowest dose at which you still feel effects). If you really need to use a higher L-Theanine dosage, we recommend taking Theanine with caffeine.  


 As with most potential L-Theanine side effects, it is difficult to find any solid research to back up the claims of nausea as a negative effect. Most people who experience nausea from using Theanine tend to be combining it with another substance like alcohol or caffeine. If you feel nauseous when you take L-Theanine, try using a lower dosage. You might also want to try taking the supplement at a different time of day and perhaps not on an empty stomach (if you took it on one and experienced nausea). Also, make sure you are not taking any L-Theanine combination with other substances.  

Potential Interactions

L-Theanine has the potential to interact with a number of medications as well as with some supplements. The most important interaction to watch for is that between L-Theanine and blood-pressure lowering medications. L-Theanine may have the potential to lower your blood pressure. Combining this with blood-pressure-lowering medication may have adverse effects.[12] Other potential L-Theanine interactions are with stimulants. However, multiple studies have confirmed that the L-Theanine caffeine combination is beneficial for cognitive performance.[13] It seems that L-Theanine has the potential to eliminate some of the negative effects of caffeine use including headaches, anxiousness, and jitteriness. L-Theanine is a relaxing supplement and it indirectly interacts with GABA receptors in your brain. For this reason, you may want to avoid using a Theanine GABA combination.  

Over-relaxation / L-Theanine high

 The so-called ‘Theanine high’ is something that many people are talking about online. This usually occurs with people taking an L-Theanine ‘megadose’ – much higher than the recommended dosage. Studies have not confirmed the long-term effects of using very high doses of L-Theanine, so we definitely don’t recommend exceed this dosage. However, from L-Theanine Reddit posts available online, it seems that very high doses of L-Theanine may result in a fairly strong sedative effect. This can be considered as being ‘overly-relaxed’. Some people complain that, at very high doses, L-Theanine has a negative effect on their ability to focus properly. [caption id=""attachment_2003"" align=""alignright"" width=""680""] L-Theanine is best known for its calming and relaxing effects[/caption]

Theanine side effects: Conclusion

In summary, L-Theanine has a few potential side effects. Like all supplements, the range of side effects experienced often depends on the user as well as the dosage. Other important factors include whether or not you are combining L-Theanine with another substance, how often you take it, and the purity or quality of the supplement that you’re using. The three most talked about L-Theanine side effects include headaches, dizziness, and interactions with medication. To avoid headaches and dizziness, make sure that you use L-Theanine at the recommended dosage and try to take it in ‘cycles’. Potential interactions include L-Theanine with stimulants, with blood pressure lowering medications, and with alcohol. It’s important to stress that L-Theanine is considered by top health and nutrition specialists to be a safe supplement that is relatively free from side-effects. Due to the factors that we’ve outlined in this article, it is possible to experience some mild Theanine side effects – as with all dietary supplements.  

Medical Disclaimer

Not intended to treat, diagnose, or cure any disease or ailment. Please read and fully understand potential adverse effects before using this product. These statements have not been reviewed by the FDA and are not written by a medical professional. Please consult your doctor before using any supplements, especially if you have any medical conditions.


B.Sc. in Molecular Biology and Biochemistry Researched & written by Tristan and verified by the Liftmode.com Research Team


[1]L-Theanine”, supplement facts on PubChem (Open Chemistry Database), US National Library of Medicine, retrieved on February 27, 2017 [2] AC Nobre et al., “L-theanine, a natural constituent in tea, and its effect on mental state”, Asia Pac J Clin Nutr. 2008;17 Suppl 1:167-8. [3] TP Rao et al., “In Search of a Safe Natural Sleep Aid”, J Am Coll Nutr. 2015;34(5):436-47 [4]Is L-theanine An Effective Anxiety Treatment?”, CalmClinic.com, retrieved on February 27, 2017 s [5]L-Theanine: Overview”, WebMD.com, retrieved on February 27, 2017 [6]Theanine”, Examine.com [7] A Yoto et al., “Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses”, J Physiol Anthropol. 2012 Oct 29;31:28. [8] FL Dodd et al., “A double-blind, placebo-controlled study evaluating the effects of caffeine and L-theanine both alone and in combination on cerebral blood flow, cognition and mood”, Psychopharmacology (Berl). 2015 Jul;232(14):2563-76 [9] LE Wise et al., “L-theanine attenuates abstinence signs in morphine-dependent rhesus monkeys and elicits anxiolytic-like activity in mice”, Pharmacol Biochem Behav. 2012 Dec; 103(2): 245–252 [10] PJ Boekema et al., “Coffee and gastrointestinal function: facts and fiction. A review”, Scand J Gastroenterol Suppl. 1999;230:35-9. [11] CF Haskell et al., “The effects of L-theanine, caffeine and their combination on cognition and mood”, Biol Psychol. 2008 Feb;77(2):113-22. Epub 2007 Sep 26. [12] A Yoto et al., “Effects of L-theanine or caffeine intake on changes in blood pressure under physical and psychological stresses”, J Physiol Anthropol. 2012; 31(1): 28 [13] T Giesbrecht et al., “The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness”, Nutr Neurosci. 2010 Dec;13(6):283-90"

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