What are The Side Effects of L-Theanine?


L-Theanine is a non-dietary amino acid found in black tea and green tea. It’s also available in supplement form to treat anxiety and insomnia. A UK study showed it improves mood, memory, and cognitive function.   

L-Theanine molecules are small enough to pass through the blood-brain barrier in half-an-hour. It restores emotional balance, promotes better sleep, induces thermogenesis (fat-burning) to encourage weight loss, reduces stress and cleanses the liver.

So what’s the catch? Any supplement with so many good qualities has to have a dark side.  Well, not really. Let’s take a look at the minimal side effects of theanine.

L-Theanine Side Effects Mayo Clinic

Although L-Theanine is used as a natural treatment for many health issues, the predominant benefits are as a nootropic (cognitive enhancer) and a gentle, natural supplement that reduces anxiety.

The Mayo Clinic recommends you talk with your doctor if you’re considering taking a supplement for anxiety. L-Theanine is considered safe for most adults (and children) with anxiety, but like other supplements, it may interfere with certain prescription medications.

If you are taking prescription anti-depressants or anti-anxiety medication, don’t take an L-theanine supplement in addition to your medicine (or switch to an L-theanine supplement) until getting the all-clear from your doctor.  

L- Theanine Side Effects Serotonin

L-theanine may decrease serotonin in the body. Serotonin is one of four neurotransmitters that carry messages along the nerves, and it’s responsible for balancing mood, along with Dopamine, GABA, and Norepinephrine.

If you’re prone to depression, it’s important to avoid any supplement, food or drink that lowers your serotonin level. While small amounts of L-theanine won’t affect most users, a dosage of 1200 mg or more a day may reduce serotonin, causing side effects, including brain fog, depression, and low energy.  Every individual is different, and you may experience reduced serotonin after taking a regular dosage of an L-theanine supplement when another person would be unaffected.  

Side Effects of L-Theanine Supplements

Serious L-theanine side effects are practically non-existent, though there are occasional minor ones. Dizziness, nausea, and headaches are the only common side effects evident in clinical studies of L-theanine.   

If you’re using Sudafed, (pseudoephedrine), epinephrine or other stimulant drugs, L-theanine may decrease the drug’s effectiveness. In one isolated case, a 38-year old woman developed blood clots in small blood vessels after drinking L-theanine rich green tea.

L- Theanine supplements may interfere with response time in some individuals. This is due to an increase in the neurotransmitter GABA L-Theanine relaxes you but doesn’t have the sedation qualities to make you drowsy. This may be a good thing if you’re highly anxious, but it may be a side effect if you’re already a laid-back type (or if you have low serotonin levels).

Reduced appetite is another “mixed blessing” type side effect. L-theanine is a natural appetite suppressant, but if you find you’re losing too much weight, decrease your dosage.  

Side Effects of L-Theanine in Suntheanine

An L-theanine supplement called Suntheanine has been available in U.S. since 2000. Suntheanine is made by a patented fermentation process and doesn’t contain any derivative of green tea leaves.  

It stimulates Alpha waves in the brain, producing the same relaxed but alert state as L-Theanine from natural tea leaves. Doses of 100 to 400 mg a day are safe for adults, but amounts over 400 mg may cause drowsiness. Doses from 400-1000 mg are probably best used to improve sleep quality and shouldn’t be taken before driving or operating machinery.

In this video, John Gray, the author of “Men Are From Mars, Women Are from Venus” talks about his experiences with Suntheanine:

Amino Acid L-Theanine Side Effects

L-Theanine is a non-essential amino acid. Amino acids are organic compounds which contain at least one carboxy group and one amino group and serve as “building blocks” for protein. Amino acids perform many functions in your body, including tissue repair, nutrient transport, and offer protection against heart disease and premature aging.  

L-Theanine alters some of the neurotransmitters in the brain, giving you a pleasant buzz and relaxing you at the same time. Think of it as coffee without the jittery effect. L-theanine actually lowers blood pressure, so if you’re taking an anti-hypertensive drug, avoid taking L-theanine supplements or take a reduced dosage. (Ask your doctor to be sure).  

The FDA has classified L-theanine as GRAS (Generally Recognized as Safe), so most people can drink green tea or take L-theanine supplements without any experiencing any side effects.

L-Theanine Side Effects Nausea

Drinking green tea makes some people nauseous, and supplements containing a high dosage of L-theanine may have the same effect. Taking L-theanine on an empty stomach, or taking a dosage that’s too high for you may cause nausea and upset stomach.

If you experience nausea after taking an L-theanine supplement, you should reduce the dosage and/or eat a meal or snack before taking the supplement again.  

L- Theanine Side Effects in Children

L-theanine supplements are used as a natural alternative to prescription medication for treatment of ADHD/ADD and insomnia in children. L-theanine is considered a safe “brain-booster” and natural treatment for hyperactivity by Vitamin Retailer.  

A study even showed 400 mg of the Suntheanine brand of L-theanine improved sleep in eight to 12-year-old boys with ADHD without significant side effects.

Children need lower dosages of L-theanine and should drink smaller amounts of tea-whether they are drinking tea for health reasons or to quench thirst, and you want to make sure they don’t drink too much. L-theanine supplements of between 50mg- 200mg daily are considered safe for children.

Give your child a small dose of L-theanine to start, and monitor their reaction to it before increasing dosage. Dizziness, nausea, headaches, and diarrhea are common side effects if the dosage is too high.  

L-Theanine is Safe for Most Users

L-theanine has no serious side effects for most people, only minor annoyances like nausea or headaches. Aside from a few drug interactions, it is safe for all users in the recommended dosage of 200 to 400 mg twice a day, with a maximum dosage of 1200 mg a day for adults.

If you’ve had any L-Theanine side effects, or if you have a question about L-Theanine, let us know in the comment section.

[catlist excludeposts=this template="div" conditional_title_tag="h6" conditional_title="Other L-Theanine resources" id="195" tags="using"]
Click Here to Leave a Comment Below 6 comments
Gail - July 23, 2017

Can this be used in conjunction with GABA or lemon balm

    Dan - July 24, 2017

    Yes to both

CJ - August 11, 2017

Does L-Theanine cause black stools and constipation?

DS - January 25, 2018

Can L-theanine increase panic attacks?

DM - December 11, 2018

How does this Magnesiumm with L-Theanine enteract with 100 mg of Sertraline daily, and Long QT Syndrome. I need help sleeping. I take Letrozol for Breast Cancer, and one of it’s side affects is insomnia. Thank you.

Dave - October 23, 2021

I’ve been using L-Theanine 100 to 200 mg at bedtime for about 2 months. I notice the benefits of slowing down the wheels in my head – which I like – but it doesn’t help me much with getting a good night’s sleep. I have to take a Benadryl for that.

Lately, however, I’ve been noticing that I feel nauseated during the day and have been having very loose stools, which is unusual for me.

So, I’m going to discontinue the L-Theanine for a while starting tonight and see if I feel better. If I can trace my unpleasant symptoms to one of my supplements (such as L-Theanine) it will put my mind at ease that there’s not something more serious going on inside me.


Leave a Reply: