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L-Theanine Side Effects & Adverse Reactions (List)

L-Theanine is a non-essential amino acid derived from the Camellia sinensis plant species.  It was first discovered circa 1950 in which it was noted within the contents of green tea; it serves as a “building block” for proteins and is also present in other types of tea (e.g. black tea).  For generations L-theanine has been utilized as a natural supplement to treat anxiety and stress, and has been investigated as a cognitive enhancer (nootropic) when administered synergistically with caffeine.

In addition to alleviating stress and enhancing cognition, preliminary research suggests additional L-theanine benefits including: anticancer properties, neuroprotective effects, sleep enhancement, and adjunctive efficacy as a treatment for schizophrenia.  In terms of structure, L-theanine closely resembles the neurotransmitter glutamate, and thus is capable of binding to glutamate receptors to a modest extent; serving as an antagonist at the AMPA and kainate receptor sites.  Further, L-theanine acts as an NMDA receptor agonist to a minor extent, while simultaneously inhibiting reuptake of glutamate and glutamine.

The substance is known to alter levels of various other neurotransmitters including: GABA, dopamine, and serotonin – plus alter neurotrophins such as BDNF and NGF.  Despite its highly complex, multi-faceted mechanism of action, L-theanine isn’t commonly associated with side effects.  In fact, most scientific literature suggests that even when administered at supratherapeutic levels, no significant side effects or adverse reactions are apparent.

Factors that influence L-Theanine side effects

Despite the fact that no significant side effects from L-theanine administration have been reported in scientific literature does not mean that they don’t exist.  Some people can still experience side effects and/or adverse reactions with this supplement.  There are several factors to consider that may influence the severity and/or number of side effects that you experience.

1. Dosage (200 mg to 1,200 mg)

As with most drugs and supplements, the greater the dosage, the greater the neurophysiological backlash you’ll experience.  Anytime you introduce a new stimuli to your body, whether it be a supplement like L-theanine or hard drug, the dosage usually dictates the severity and quantity of side effects.  Generally those at the higher end of the dosing spectrum will get a greater effect compared to those at the lower end of the spectrum.

Someone taking 200 mg of L-theanine is unlikely to experience any significant side effect, yet someone taking over 1,200 mg will be more likely to notice side effects.  Sometimes the ratio of dose to bodyweight is an influential factor.  Just like some bigger individuals can tolerate copious amounts of alcohol, smaller statured individuals may not be able to handle as large of a dose.

While the concept of dose in relation to bodyweight is always used in mice and rodent studies, it is seldom advised for humans – despite the fact that it is a proven paradigm.  To minimize the likelihood of side effects from L-theanine supplementation, you’ll want to take the minimal effective dose.  In other words, strive to take the amount that gives you therapeutic benefit, yet yields few (or no) side effects.

2. Individual variation

Another important factor to consider when experiencing side effects is individual variation.  Why do some people experience side effects from L-theanine, yet others only notice benefit?  It is important to consider the influence of genetics, lifestyle, environment, stressors, neurophysiology, and arousal when considering the side effects a person experiences.

Two people could be administered the same dose of L-theanine, yet one person may feel increasingly groggy or feel cognitively impaired – while the other feels more relaxed and alert.  The difference between these two individuals could be related to certain genetic biomarkers that interact with the supplement or other stressors.  Although certain drug reactions can be predicted in advance with technologies like GeneSight – this doesn’t yet apply to supplements.

In addition, your unique neurochemistry (neurotransmitters, brain waves, brain structure, regional activation) will all dictate your response to L-theanine. Some people will report favorable, noticeable responses – yet others may experience side effects.  Due to the fact that L-theanine isn’t associated with many side effects, individual variation may be a strong influential factor.

3. Interactions

This ties in with the variable of individual variation, but it is necessary to consider potential interactions between L-theanine and other drugs (or supplements).  If you’re a heavy drinker of alcohol and supplement with L-theanine, it is important to consider the fact that these substances could interact.  Individuals that are taking pharmaceutical drugs, ingesting illicit drugs, or even taking other supplements may experience side effects stemming from interactions.

It is important to consider the contraindications associated with L-theanine and other substances prior to supplementation.  Many speculate that L-theanine may interact most strongly with CNS depressants such as alcohol and especially anxiolytic sedatives (e.g. benzodiazepines).  Some evidence suggests that L-theanine elicits synergistic effects with these substances.

4. Time Span + Frequency

The duration over which you use L-theanine as a supplement may influence the side effects that you experience.  Someone that has taken L-theanine for a few days may experience short-term effects as their body learns to adapt to the physiological changes being made by the substance.  Others may not experience short-term effects, but may find that side effects emerge as they take L-theanine over a longer term.

In addition to time span (duration over which you’ve used L-theanine), it is important to consider the frequency of usage.  Someone who administers L-theanine several times per day, on a daily basis may develop certain tolerances to the substance – eventually leading to them taking higher doses.  As a person increases their dosage into the supratherapeutic range, the prevalence of side effects may increase.

To minimize the likelihood of side effects that you experience, consider taking L-theanine for short periods and cycling off.  Limit the frequency of supplementation to an “as needed” basis to minimize likelihood of building tolerance, ultimately allowing you to maintain a low dose, which generally reduces potential side effects.

5. Specific Source (Brand)

While most L-theanine supplements are similar, being similar does not necessarily mean they are identical in composition.  It is important to consider the brand and/or source of the L-theanine that you purchase.  For example, some people purchase a specific “brand name” version of L-theanine called “Suntheanine” which contains a specific percentage of the “L” levorotatory enantiomer compared to the “D” dextrorotatory enantiomer.

Other sources may market their supplement as “L-theanine” but have more of a racemic mixture of the L-Theanine and D-theanine.  In addition to the racemic composition, it is important to consider variation in quality between brands.  Certain brands may manufacture L-theanine with varying amounts of herbal extracts or other trace elements.

Some supplement companies do a better job of manufacturing quality L-theanine than others, and many individuals will report subjective differences in “efficacy” between certain brands – which could be influenced by manufacturing quality.  While the specific source of L-theanine is relatively minor, it should not be assumed that every batch is the “exact” same.  This means that specific sources or brands may trigger more side effects than others for certain individuals.

L-Theanine Side Effects & Adverse Reactions (List)

Most medical professionals and researchers suggest that L-theanine supplementation isn’t associated with any significant side effects nor adverse reactions.  In addition, the FDA considers L-theanine to be a GRAS (Generally Recognized As Safe) supplement.  Despite the fact that there isn’t significant scientific evidence to suggest side effects from L-theanine doesn’t mean that they never occur.

Below are some of the more commonly reported side effects from L-theanine supplementation.  Of the side effects on this list, the most common tend to include: dizziness, gastrointestinal discomfort, and headaches.

  • Appetite changes: Some people have noted that when taking L-theanine, they experience appetite changes. For many individuals, these appetite changes will not be substantial, but may be noticeable.  L-theanine within green tea and/or when combined with caffeine may aid in the suppression of appetite.
  • Blood pressure reduction: There is some evidence to suggest that L-theanine can reduce hypertension and/or attenuate stress-induced blood pressure increases. It is unknown as to whether a blood pressure reduction may also occur among individuals with normal blood pressure.  Individuals taking antihypertensive drugs or with lower blood pressure may want to be cognizant of this potential side effect.
  • Brain fog: L-theanine as a standalone supplement is not a scientifically-proven cognitive enhancer. Certain studies have shown that it actually decreases cognitive performance on a variety of tasks when administered in isolation.  If you find yourself getting brain fog from L-theanine, adding some caffeine to the equation should reverse this side effect.
  • Brain wave alterations: It is scientifically proven that L-theanine alters brain waves, specifically increasing alpha waves. This effect is most noticeable among individuals who are stressed and/or anxious.  Individuals with excess alpha activity and/or problems generating sufficient beta waves may perceive this as an unwanted side effect; for most people increased alpha should be considered beneficial.
  • Cognitive slowing: When taken at high doses in isolation, L-theanine may impair various aspects of cognition, leading to slowed cognition. If you feel as if your cognitive function is impaired following administration of L-theanine, there’s a possibility that it has been.  To prevent cognitive slowing from L-theanine and to enhance your cognitive function, take it with caffeine (or another stimulatory agent).
  • Diarrhea: Some people experience diarrhea when they first start taking L-theanine. Diarrhea may be indicative of a poor quality brand of L-theanine or simply could be related to gastrointestinal distress.  To reduce the likelihood of diarrhea, you have a number of options including: scale back on dosing, switch brands, take Imodium, discontinue, or tough it out and see if the diarrhea subsides.
  • Dizziness: Among the most common side effects from L-theanine supplementation is dizziness. If you start taking this supplement and feel as if you’re too dizzy to function, feel semi-drugged, or even drunken – you may have taken too high of a dose.  To minimize the likelihood of dizziness, start with a low dose and realize that the dizziness may subside as your physiology adapts to the L-theanine.
  • Gastrointestinal discomfort: Another common side effect of L-theanine is gastrointestinal discomfort and/or pain. If you notice that your digestive system is acting up and/or you feel stomach pain, it could be from the supplementation.  To minimize this side effect, you’ll want to consider taking it with food and/or at a lower dose.
  • Headaches: Many people report headaches from L-theanine, which is relatively counterintuitive because the supplement increases relaxation. A majority of headaches associated with supplementation occur for a few days, and diminish as a person’s body adjusts to the effect of the supplement.  The headaches stemming from L-theanine may be related to its complex effect on nitric oxide, which is known to cause headaches.
  • Low energy: Taking L-theanine may promote increased physical and mental relaxation. While increased relaxation is nice, some individuals may note reduced overall energy as a result.  If L-theanine is making you feel increasingly sluggish, you could augment its supplementation with caffeine and/or reduce the dosage.  Understand that reductions in energy may be temporary and associated with neurophysiological adjustments to the supplement.
  • Low serotonin: L-theanine has been suggested to alter neurotransmission of serotonin – some evidence suggesting increases, while other studies suggesting decreases. The evidence suggesting decreases note that while L-theanine increases levels of tryptophan (a precursor to serotonin), it reduces cortical levels of serotonin (5-HT) and decreases serotonin synthesis (5HIAA). Among certain individuals, a decrease in serotonin and/or serotonergic adjustment may serve as an unwanted side effect.
  • Nausea: It is possible to feel nauseated following supplementation with L-theanine. This nausea may be more common when administered on an empty stomach, but may also be a sign that your body is not well-adapted to accommodating large doses.  To minimize the likelihood of feeling nauseous, start with a low dose and titrate upwards.
  • Sleepiness: While L-theanine doesn’t seem to cause drowsiness and/or somnolence in most individuals, it may synergistically exacerbate sleepiness among individuals that are already tired, fatigued, and/or taking a CNS depressant (e.g. alcohol). It is also important to consider that when taken at extremely high doses, its anxiolytic effect may be potent enough to provoke sleepiness in certain individuals.
  • Stomach aches: It is very common to develop stomach aches following L-theanine supplementation. L-theanine is known to cause gastrointestinal distress and may provoke diarrhea.  If you notice that you’re feeling stomach pain after taking L-theanine, you may want to consider taking it with food and/or reducing the dosage until your body adapts to its effect.
  • Vomiting: In rare cases, nausea from L-theanine may become significant enough to provoke vomiting. Those that vomit from L-theanine are most likely to either have: taken too high of a dose, taken it on an empty stomach, purchased from a low quality brand, and/or taken it with another substance (spurring an interaction).  If you’re certain that L-theanine is the culprit for your vomiting spell, discontinuation is advised.
  • Weight loss: Preclinical evidence (from rodent studies) suggests that L-theanine may elicit anti-obesogenic effects. In other words, it could help reduce the likelihood of weight gain, and may even promote weight loss.  While L-theanine isn’t often regarded as a weight loss drug, it is theoretically plausible that weight loss may result (in certain individuals) from supplementation.

Note: It is important to reemphasize that side effects are subject to significant individual variation.  Also, it is necessary to understand that those who experience side effects from L-theanine are unlikely to experience every aforementioned possibility.

  • Source: http://www.ncbi.nlm.nih.gov/pubmed/18006208

L-Theanine: Analyzing the Benefits (Pros) and Side Effects (Cons)

Anytime you’re taking a supplement like L-theanine, you should conduct a cost-benefit analysis to determine whether the therapeutic benefits (pros) outweigh the side effects (cons).  Since this supplement isn’t associated with any clinically significant side effects, most people find that the benefits significantly outweigh the drawbacks.  For example, someone with high anxiety and/or who experiences stress-induced cognitive impairment may derive significant benefit from L-theanine.

Even if this individual experiences a minor side effect such as slight nausea or an upset stomach, the therapeutic benefit outweighs these side effects.  In other cases, it may be more difficult to determine whether the benefits outweigh the drawbacks.  For example, if you’re taking L-theanine for anxiety, but find that it’s providing minimal benefit and making you feel really dizzy – it may be worth discontinuing.

In other scenarios, certain individuals may find no significant benefit and experience no significant side effects.  In this case, it may be worth stopping L-theanine to save yourself the expense of buying supplements.  It should be noted that individuals prone to anxiety and stress often derive more benefit from L-theanine than individuals who are naturally more laid back and less stressed; this has been noted in research.

Have you experienced L-Theanine side effects?

If you’ve supplemented with L-theanine, feel free to share whether you experienced any significant side effects in the comments section below.  To help others understand your situation, document the dosage of L-theanine you had been taking, the specific brand, and how frequently you take it.  In addition, note when the side effects emerged and/or if they ever subsided.

To those that experienced side effects, how can you be sure it was from the L-theanine?  Were you only taking L-theanine or ingesting other drugs and supplements?  Finally, include whether you believe the benefits associated with L-theanine supplementation were substantial enough to outweigh any side effects.

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{ 24 comments… add one }
  • Ranjith November 1, 2015, 4:48 pm

    I took my first ever dose of L-theanine, a single cap of Source Naturals Theanine Serene, hoping to reduce anxiety that I get once in a while when I’m overwhelmed with work. Recommended dose is 2 caps but started with one and in the afternoon got this really bad headache, and tried to get some rest. This might be my body adjusting to the new chemical, will see if this persists.

  • Jack December 16, 2015, 4:15 am

    I took one pill from a bottle a guy at GNC recommended to me. Unknown to me, the bottle was pharmaceutical grade. I took one pill and proceeded to throw up, feel dizzy, etc. for 3 hours after ingesting. After a couple of vomiting spells, I made myself vomit until I got everything out that I thought I could. By far, one of the worst experiences I have ever had – and I have had bad trips before. I will never touch this stuff again.

  • Lisa January 10, 2016, 1:12 pm

    Theanine has been wonderful for me in reducing stress and anxiety. I try to cycle off it at times starting at 100mg twice a day up to 200mg three times a day and have never had any side effects. I’ve found it a great aid in avoiding medication while pairing it with alternating every so often between rhodiola and holy basil.

  • Germaine January 14, 2016, 11:11 pm

    I started taking Suntheanine, a patented form of L-theanine, this week and it works wonders for me so far. I have been suffering from anxiety and depression (I suppose so) and all of a sudden, I was able to sleep and wake up as I wish; my energy level and especially my cognitive thinking function seems to recover back to the old days when I could focus and study. Before this, I seriously thought I was getting real dumb.

  • Pamela March 2, 2016, 11:08 pm

    I tried this last night for anxiety and stress. My anxiety is really bad. It worked but it made me extremely drowsy. It helped my anxiety a lot but I do not like feeling this drowsy. Has anyone had this experience?

    • John April 25, 2016, 11:51 pm

      Try taking it with caffeine. They work well together.

  • Dee April 3, 2016, 6:21 pm

    Brilliant article thank you. I found it very helpful and educational.

  • PG April 13, 2016, 1:54 am

    A family member took 100 mg of suntheanine in the afternoon and felt almost instantly sluggish in an uncomfortable way. Rather than the post work exercise they do daily, they came home and went immediately to sleep. Definitely not the reaction we were expecting. This person does drink alcohol recreationally but did not have any the day of this dose or the day prior.

  • Susan April 15, 2016, 9:21 am

    Thorough and informative article. Thanks!

  • Candace June 15, 2016, 6:49 pm

    I’ve been taking 100mg once a day for the past two days for anxiety. I feel calm after 20 mins or so but I start feeling slightly dizzy and “off” about 3 hours after taking it. I’m not sure if my body is adjusting to it or if it’s more of an adverse reaction. I take Xanax as needed but was really hoping this could replace the Xanax. I don’t like how this makes me feel after a few hours as it increases my anxiety. Has this happened to anyone else?

  • Jack Meme June 17, 2016, 12:35 am

    Candace – same thing happens to me. I’ve been taking it for a while and eventually the side effects lessen, but if I were you I’d stay at your current theanine dose and continue for a while, or maybe even reduce it to 50mg and keep at that until the side effects begin to diminish. The worst it can do is make you feel nauseous and maybe be sick (rarely), you’ll do no damage to yourself permanently.

  • Sarah July 10, 2016, 6:17 pm

    I took 2 L Theanine pills as directed on the bottle and about 3 hours later I started to feel unwell and started vomiting regularly for 4 hours – and couldn’t keep fluids down. Ended up having to go to ER for fluids. Not sure whether to try half a pill and work my way up to see if it was the culprit or just coincidence.

  • GDub July 26, 2016, 4:46 am

    Been taking 25mg of L-Theanine as part of a supplement mix containing Melatonin, Valerian and GABA, as a sleep and hot flash aid. Melatonin itself doesn’t work for me, but this supp works like a dream. I discovered L-Theanine when it was an ingredient in a dog biscuit for anxious dogs. That worked well for my dog too. Interesting ingredient and it’s being mixed with other supps for different needs.

  • Liz August 12, 2016, 3:25 am

    I began taking Twin Labs chew tabs of l-theanine as needed. I paid no attention to whether I had food in my stomach or not. Just took as needed for stress/anxiety. It wasn’t daily but I can’t remember how frequent it was. Wasn’t crazy amounts. Once or twice a day at most.

    Started having really bad stomach aches. Nauseous for weeks. All day every day for weeks. Turned into months. I stopped taking them as a first step to figuring out what my stomach aches were from. I haven’t had the nausea spells since. I’m wondering if it was from them.

  • CHRISTINA HASLAM August 29, 2016, 11:41 am

    I took 200mg of l theanine once and got diarrhea the day after. I am also on antidepressants so I will not take it again. I also have been taking ashwagandha for several weeks and it worked brilliantly to calm me down. But it is an adaptogen which means that once I was calm it began to speed me up, this resulted in severe diarrhea. So be very careful with this.

  • Ann September 4, 2016, 6:58 pm

    Tried L-theanine and it causes horrible muscle spasmodic episodes in my back and legs. Waited a week and tried again with even worse consequences, it triggered sleep paralysis and almost charley horse knots, this was two days after being glutened (been gluten free 8 years, as it causes Narcolepsy in me) the Brand was expensive Integrative medicine prescribed by my Naturopath! Headache and moodiness follow for a few days! I won’t be taking again!

  • Sienna September 8, 2016, 6:01 am

    I recently purchased a supplement that is an energy supplement that also helps one to focus (contains caffeine and l-theanine and theacrine). The first time I took it, I thought maybe I was coming down with something because I felt terrible. I took it again another day and thought, again, maybe I was coming down with something.

    I wanted to take a nap, it did not give me energy and I had a headache. I took it a third time, when I knew I was feeling healthy, and again, so tired and felt like crap all day with a headache and nauseous. I will never take it again. My body rejects it.

  • Helen September 11, 2016, 4:19 am

    I noticed at work after a couple of days of taking my Adderall, coffee and drinking some cold diet green tea I felt really good. The only thing I added was the green tea so I started researching and learned about the L -theanine. I have fibromyalgia, ADD (no focus or organization), mild depression and tend to sleep way too late on days that I don’t have to work… don’t wake up natural after 8 hours but more like 10 hrs.

    I take Adderall, luvox and Neurontin but would like to feel better. My meds help some. One of my sleeping problems is that I have very vivid dreams and love to sleep… like going to the movies! My BP is sometimes as high as 140/88 but is usually around 130/76. I decided to start taking L-theanine to feel better.

    I was hoping to wake up feeling refreshed and more focused during the day. I bought some Sundown naturals stress formula from Walmart which is L-theanine. I started taking only 100 mg one morning and had some coffee with it. I didn’t really feel that different but then took it at night. I woke up not feeling that great because I slept my day away.

    I slept for 12 hours straight and I think the only reason I woke up was because I had to go to the bathroom. I dreamed all night long. My lower back was hurting and I didn’t feel rested. A few hours later I took my BP and it was 96/64. I felt sluggish most of the day.

    I noticed that according to bottle a serving is 2 pills which would be 200 mg. I am taking that tonight as I don’t have to work tomorrow and I would like to see what happens. I use an eye drop for glaucoma which is a beta blocker and I use in the AM but from what I read I don’t think that is a problem.

    I’m going to take this for 3 more days and in the mornings take with coffee and see how things go. Hoping this is just my body getting used to it. May have to go back to just some cold green tea with my coffee in the mornings instead of this. Will wait and see.

  • Catpainter67 September 23, 2016, 6:52 pm

    I took my first dose of pure l-theanine last night – 1/2 of a 200mg tablet along with 100mg pure GABA for overactive brain at night. Got drowsy after 45 min and slept soundly for 7 hours. Upon awakening, I was full of energy and, as day wore on became more hyper and very slightly dizzy. My only prescription drug is a low dose statin. I take vitamins in the AM.

    I took my blood pressure and it had dropped about 15-20 pts from it’s regular low-normal range and pulse had increased by 15 pt. Low BP could be causing light dizziness. Don’t think I will take any more of either.

  • Bethany J Fowler September 28, 2016, 12:16 am

    I’ve been taking it for a month and a half. It’s been a beautiful experience. It helped me quit smoking. I go straight to sleep at night. I feel like I did on shrooms, very verbal and alert of my own thoughts and feelings.

    I work two jobs and this has helped me zoom through the day. I already have low blood pressure haven’t checked my current pressure but I haven’t felt dizzy. And I have gotten drunk while taking it and no effects. I don’t get hangovers anyways. Helps PMS go away.

  • Morgan October 8, 2016, 11:21 pm

    I recently was given L-theanine by someone who works at a vitamin store, he told me it’ll help with my anxiety. I took it, as directed which is 2 200mg, this morning and didn’t feel a reduction for my anxiety. Now that it’s almost 7pm and my anxiety is still getting the best of me, is it possible to take my prescription .25mg Xanax or will the two interact in some way?

  • Yolanda Chavira October 10, 2016, 2:39 am

    Given to Teenager with mood disorder, depression, mild tourettes & insomnia. Only took for 3 days L-theanine 25 mg, 3 mg melatonin which was a supplement that included B6-10 mg, calcium 100-mg & phosphorus 80-mg. Worked immediately on all his problems. Vomiting on the third day, once, but not sure if it was the flu as he was sick with fever before starting supplement & he said his skin on his chest was extremely sensitive. Has been feeling very positive & calm since. Not sure if we should continue…

  • Ant October 11, 2016, 11:56 pm

    L-theanine works great for my focus and anxiety. The only downside is that I get a weird lightheaded feeling a few hours and taking 200mg of Suntheanine. I’d like to continue using L-theanine, but not if I can’t find a way to get rid of the light headed feeling. Any ideas?

  • IMP October 20, 2016, 1:03 am

    I started taking Theanine Serene for anxiety about 3 weeks ago, and almost immediately noticed a significant decrease in overall body aches & pains. Appetite has not changed. The only negative side effect, is decreased thirst and the resulting dry skin and SEVERELY chapped lips I can cover in balms and emollients, but no joy.

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