Does L-Theanine Cause Headaches? What the Research Shows

Usually not. Headache is rarely reported in L-theanine trials, and the best-known signal came from one study that gave 250 mg on its own, five times the amount in one of our capsules. When a headache follows a new caffeine and L-theanine supplement, the caffeine is the more likely cause, either too much of it or less than your body is used to. Here is what the trials report, the likely reasons and their evidence, what to try, and when to see a physician.

Can L-theanine cause headaches? What the trials report

The one clear signal comes from a 2008 placebo-controlled crossover study in volunteers that tested 250 mg of L-theanine, 150 mg of caffeine, and the two together. Taken alone, L-theanine raised headache ratings compared with placebo. Taken with caffeine, headache and tiredness ratings went down. These were self-rated scores during a few hours of lab testing, not a count of people who developed headaches.

Longer trials have not repeated it. In a 2024 trial of 30 healthy adults taking 400 mg of L-theanine a day or placebo for 28 days, the only headache reported was in the placebo group. A 2019 trial of 30 adults taking 200 mg a day for four weeks reported no apparent adverse events, and a 2026 meta-analysis of 31 randomized trials with 1,168 participants found no serious adverse events.

So headache with L-theanine is possible, but it is uncommon and poorly documented.

Why does L-theanine cause headaches? The likely reasons

Some have good evidence. Others are educated guesses.

Well supported: caffeine withdrawal after a swap

This is the first thing I look for as a physician. People often start a capsule in place of coffee, and if that means less caffeine than usual, withdrawal can follow. A 2004 review of caffeine withdrawal research found headache in about half of people in experimental studies, usually starting 12 to 24 hours after the last dose, peaking at 20 to 51 hours and lasting two to nine days, at habits as low as 100 mg a day.

Caffeine narrows blood vessels in the brain. In a 2000 study of 10 regular caffeine users, about a day without caffeine sped up blood flow in major brain arteries, a change the researchers suggested may be related to withdrawal headache. One capsule contains 22.5 mg of d9-caffeine, which produces four to five times the total exposure of a matched dose of regular caffeine, but no study has tested how much coffee one capsule replaces. If you went from three cups to one capsule and the headache arrived the next day, withdrawal is the prime suspect.

Well supported: too much total caffeine

Headache is on the FDA list of signs of too much caffeine, along with jitters, a faster heart rate and trouble sleeping. With migraine, the amount may matter: in a 2019 diary study of 98 adults with episodic migraine, one or two caffeinated drinks a day were not linked with more attacks, while the odds trended higher at three or more, though no level was statistically significant alone.

Possible: L-theanine itself at a higher dose

The 2008 signal appeared at 250 mg taken without caffeine. Some people may react to L-theanine directly, but no trial has shown how, and the longer trials at 200 to 400 mg a day did not repeat it.

Less supported or untested

• Blood pressure. In a 2008 study of 48 adults, 200 mg of L-theanine blunted the blood pressure rise from 250 mg of caffeine, and a 2012 study of 14 adults found it softened blood pressure rises during stressful mental tasks in strong responders. No study has linked these modest effects to headaches.
• Dehydration. In a 2014 study of 50 men who habitually drank coffee, four cups a day hydrated as well as water, so a 22.5 mg capsule is an unlikely culprit. In a 2012 trial of 102 adults with recurrent headaches, drinking more water improved headache-related quality of life, though not the number of headache days.
• The glutamate idea. L-theanine is structurally related to glutamate, so some people compare it with MSG. I found no study that tests that link, and a 2016 systematic review found that studies giving MSG with food mostly showed no difference in headaches.
• Other ingredients. Some blends add extra caffeine or other stimulants, so read the label. Ours contains d9-caffeine, L-theanine, microcrystalline cellulose and magnesium stearate in a cellulose capsule.

How our 50 mg dose compares with studied doses

One capsule of Caffeine 2.0 + L-Theanine has 50 mg of L-theanine, and the daily maximum of four capsules provides 200 mg across the day. Compare that with:

• A cup of tea: a 200 mL cup of green or black tea typically contains about 8 to 25 mg of L-theanine, according to a 2026 review.
• The headache study: 250 mg in a single dose, without caffeine.
• The longer trials: 200 to 400 mg a day for four weeks, with no headache signal.
• The regulatory reference: in 2007 the FDA had no questions about a GRAS notice for L-theanine at up to 250 mg per serving.

A single capsule sits at the low end of what has been studied, and it always comes with caffeine, the condition in which the 2008 study saw headache ratings fall rather than rise. That one lab result does not show that the pair prevents or treats headaches. For the wider safety picture, see L-theanine and caffeine side effects.

What to try if you get a headache

• Count all your caffeine. Capsules, coffee, tea, soda, energy drinks and pre-workouts all add up. The FDA cites 400 mg a day as an amount not generally linked to negative effects in healthy adults, and sensitivity varies widely. See how much caffeine is too much.
• If you cut back on coffee, step down gradually. Dropping one cup at a time over a week or two is gentler than swapping everything on day one. Our guide to caffeine withdrawal and how to taper walks through it.
• Drop to one capsule, in the morning. Start low, as the label directs, and watch whether the headache tracks the dose. d9-caffeine lasts longer than regular caffeine, so keep doses early.
• Keep a three-day log. Note dose times, other caffeine, sleep, meals, fluids and when the headache started.
• Pause and reassess. If headaches continue, stop and see whether they clear. After weeks of several capsules a day, step down over a few days rather than stopping at once.

When a headache needs a physician

Most caffeine-related headaches are mild and fade within days. Get emergency care for a sudden, severe headache that peaks within about a minute, a headache with fever, a stiff neck, confusion, weakness, numbness, trouble speaking or vision changes, or one that follows a head injury. See your physician soon for a new headache pattern or one that keeps getting worse, headaches triggered by coughing, sneezing or exercise, headaches that change when you stand up or lie down, new headaches during pregnancy or after giving birth, new headaches after age 65, or needing pain relievers most days.

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 migraine or frequent headaches, 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, never exceed 90 mg of d9-caffeine, and avoid late-day doses. Count it as caffeine when you add up your day.

Frequently asked questions

Can L-theanine cause headaches?

It can in some people, but it is uncommon. The clearest signal is a 2008 study in which 250 mg taken alone raised headache ratings; longer trials at 200 to 400 mg a day did not show a headache problem.

Why does L-theanine cause headaches in some people?

No mechanism has been proven. When the product also contains caffeine, the usual culprits are caffeine withdrawal after cutting back on coffee, or too much total caffeine.

Is my headache from L-theanine or from caffeine withdrawal?

Withdrawal headaches typically start 12 to 24 hours after you cut back and fade within two to nine days. If yours began right after swapping coffee for a lower-caffeine product, withdrawal is the more likely cause.

Should I stop taking L-theanine if I get a headache?

First drop to one morning capsule and check your total caffeine. If headaches persist, stop and see your physician, and seek care right away for any of the warning signs above.

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.