L-Theanine Side Effects: What to Know, Including With d9-Caffeine
In this article
L-theanine has a reputation as one of the gentlest supplements on the shelf, and for most healthy adults the research supports that. Gentle is not the same as effect-free, though. Here is what is known about L-theanine's side effects, who should be careful, and what changes when it is paired with d9-caffeine.
What L-theanine is
L-theanine is an amino acid found naturally in tea leaves. In 2007 the FDA responded with no questions to a GRAS notice (GRN 209) for L-theanine at up to 250 mg per serving, and other GRAS notices have followed. Human studies commonly use doses between about 50 and 250 mg.
Reported side effects
Headache
In a 2008 study, L-theanine taken on its own raised headache ratings compared with placebo, while L-theanine combined with caffeine lowered headache and tiredness ratings.
Drowsiness
L-theanine is often described as relaxing, and some people report feeling drowsy. Until you know how it affects you, avoid driving or operating machinery right after a dose.
Stomach upset
Mild nausea or stomach discomfort is occasionally reported, as with many supplements. Taking it with food may help.
Blood pressure
In a 2008 study, L-theanine blunted the rise in blood pressure that caffeine produced. For most people that is unremarkable, but if you take blood pressure medication, talk to your physician, since the effects could add together.
| Possible effect | What is known | What to do |
|---|---|---|
| Headache | Higher ratings with L-theanine alone in one study, lower with caffeine | Stay hydrated; stop if headaches persist |
| Drowsiness | Reported by some users | Learn your response before driving |
| Stomach upset | Occasional and usually mild | Take with food |
| Blood pressure | Blunted caffeine's blood pressure rise in one study | Check with your physician if you take blood pressure medication |
Who should be careful
- Pregnant or breastfeeding women: there is not enough safety data, so avoid it.
- People on blood pressure medication: talk to your physician first.
- People on sedatives, sleep medication or anti-anxiety medication: calming effects could add together, so talk to your physician first.
- Anyone under 18.
What changes when it is paired with d9-caffeine
When L-theanine is paired with d9-caffeine, the side effects that matter most are the caffeine ones: restlessness, a racing heart, stomach upset and trouble sleeping. d9-caffeine is caffeine with nine hydrogen atoms replaced by deuterium, which slows how quickly it is broken down. In a 2022 double-blind study in 20 healthy adults, it produced four to five times the total exposure of the same molar dose of regular caffeine, and it was well tolerated.
That makes timing the most important safety habit: take it in the morning so it does not reach into your sleep, and do not stack it with coffee, energy drinks or pre-workouts. Four capsules of Caffeine 2.0 + L-Theanine, the daily maximum, contain 90 mg of d9-caffeine and 200 mg of L-theanine, which keeps L-theanine within the 250 mg per serving covered by its GRAS notice. Our guide on dose, timing and sleep covers the schedule.
Who should check with a physician first
Skip Caffeine 2.0 + L-Theanine, or talk to your physician first, if you are pregnant or breastfeeding, under 18, sensitive to caffeine, or have a heart condition, high blood pressure, an arrhythmia, an anxiety disorder or a sleep disorder. Check with your physician or pharmacist if you take prescription medication, especially stimulant medication, blood pressure medication or sedatives.
How Vita Bloom fits
Caffeine 2.0 + L-Theanine pairs 22.5 mg of patented Deura9® d9-caffeine with 50 mg of L-theanine in every capsule. Start with one capsule in the morning, take up to four a day, and never exceed 90 mg of d9-caffeine. Prefer to skip the L-theanine? Caffeine 2.0 is d9-caffeine on its own.
Frequently asked questions
Is L-theanine safe to take every day?
For most healthy adults, studies show it is well tolerated, and the FDA had no questions about its GRAS status at up to 250 mg per serving. Most studies are short, so follow your label.
Can L-theanine cause headaches?
In one 2008 study, L-theanine alone raised headache ratings, while L-theanine combined with caffeine lowered them.
Does L-theanine make you sleepy?
Some people report drowsiness. Learn how it affects you before driving.
Can I take L-theanine with blood pressure medication?
Talk to your physician first, since L-theanine may influence blood pressure responses.
How much L-theanine is too much?
Its GRAS notice covers up to 250 mg per serving. Caffeine 2.0 + L-Theanine contains 50 mg per capsule, up to 200 mg a day.
Deura9® is a registered trademark of Lennham Pharmaceuticals, Inc. under exclusive license to d9 Designs Inc.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
Educational information, not personal medical advice. Ingredient research does not establish that a finished supplement treats or prevents a disease. Follow your product label and ask your healthcare professional about personal suitability.
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