L-Theanine and Caffeine Side Effects: What the Pair Can Cause

Most side effects of the L-theanine and caffeine pairing come from the caffeine: jitters, a racing heart, stomach upset, headache and poor sleep. L-theanine on its own is well tolerated in trials, and although it blunted caffeine's rise in blood pressure in one study, it has not been shown to reliably prevent caffeine's other side effects. Here is which ingredient causes what, what the combination trials found, key interactions, and how d9-caffeine changes the timeline.

Caffeine side effects: where most problems start

Caffeine is the stimulant in the pair, so it accounts for most of what people feel. The FDA cites 400 mg a day, about two to three 12-ounce cups of coffee, as an amount not generally linked to negative effects in healthy adults, while noting that sensitivity varies widely. Signs of too much, according to the FDA, include a faster heart rate, palpitations, higher blood pressure, trouble sleeping, anxiety, jitters, upset stomach, nausea and headache.

In a 2008 study of 48 adults, 250 mg of caffeine raised alertness but also jitteriness and blood pressure. In a 2013 study of 12 healthy sleepers, 400 mg of caffeine taken six hours before bed cut objectively measured sleep by more than an hour. Daily caffeine also brings withdrawal if you stop, with headache, fatigue and poor concentration usually starting 12 to 24 hours after the last dose.

Count Caffeine 2.0 + L-Theanine as caffeine: d9-caffeine is still caffeine, just broken down more slowly. See how much caffeine is too much for daily totals.

L-theanine side effects: a short list

L-theanine is an amino acid from tea, not a stimulant, and its record in trials is short:

• Few problems overall. A 2026 meta-analysis of 31 randomized trials with 1,168 participants found no serious adverse events, and a 2026 review of sleep trials using 50 to 1,000 mg a day for up to eight weeks described few, mild adverse effects.
• Mild effects in one small trial. In a 2024 trial of 30 healthy adults taking 400 mg a day for 28 days, the events judged possibly related to L-theanine were mild dry mouth and a metallic taste.
• Headache ratings at a high solo dose. In a 2008 crossover study, 250 mg of L-theanine alone raised headache ratings, while the same dose with 150 mg of caffeine lowered them. See does L-theanine cause headaches for the full picture.
• Slower reactions, sometimes. In the 2008 study of 48 adults, 200 mg of L-theanine slowed overall reaction time on one attention task. Reviews describe it as generally not sedating, but learn how it affects you before driving.

In 2007 the FDA had no questions about a GRAS notice for L-theanine at up to 250 mg per serving. Four capsules of Caffeine 2.0 + L-Theanine, the daily maximum, provide 200 mg across the day.

What the combination studies found

The best-known caffeine and L-theanine trials were short, single-dose lab studies built to measure attention and mood, not long-term safety:

• 2008, 27 adults: 50 mg of caffeine with 100 mg of L-theanine improved the speed and accuracy of attention switching.
• 2010, 44 young adults: 40 mg of caffeine with 97 mg of L-theanine improved task-switching accuracy and alertness and reduced tiredness.
• 2008, 150 mg of caffeine with 250 mg of L-theanine: the combination lowered headache and tiredness ratings.
• 2014 meta-analysis of 11 trials: moderate benefits for alertness and attention switching in the first two hours, with the caffeine dose appearing to matter more than the L-theanine dose.

Not every study agrees. In a 2015 study of 24 adults, 50 mg of L-theanine added to 75 mg of caffeine cancelled caffeine's narrowing of blood vessels in the brain, but it also erased caffeine's measurable effects on attention and mood 30 minutes after the dose.

Does L-theanine reduce caffeine jitters or blood pressure?

Blood pressure: some evidence, yes. In the 2008 study of 48 adults, 200 mg of L-theanine counteracted the blood pressure rise from 250 mg of caffeine. A 2012 study of 14 adults found that L-theanine softened blood pressure rises during stressful mental tasks in strong responders.

Jitters: mixed. The same 2008 study found that L-theanine did not significantly change caffeine's jitteriness or other mood effects. A 2025 crossover study in 12 elite male wrestlers pointed the other way: 92 percent reported a fast heart rate after caffeine alone, compared with 17 percent when L-theanine was added, and anxiety scores were lower with the pair than with caffeine alone. That is one small study in young athletes, using doses set by body weight.

In short, L-theanine may take the edge off for some people, but plan as if caffeine side effects are still possible. Our guide to caffeine jitters covers practical fixes.

Interactions to know about

• Stimulant medications. A widely used drug interaction reference rates caffeine with amphetamine as a moderate interaction, because both raise blood pressure and heart rate and the effects can add up. Ask your prescriber.
• Blood pressure medication. Caffeine raised blood pressure in the 2008 study, and L-theanine blunted that rise. The pair has not been studied with blood pressure drugs, so talk to your physician and check your readings when you start.
• Sedatives and sleep medication. Memorial Sloan Kettering lists sedatives among its contraindications for L-theanine and notes it may add to the drowsiness of sleep medications such as lorazepam, diazepam and zolpidem.
• Drugs that slow caffeine breakdown. Some medications block CYP1A2, the liver enzyme that clears caffeine. In a 1996 study of eight healthy volunteers, the antidepressant fluvoxamine stretched caffeine's half-life from about 5 hours to 31. In a 1989 study of 10 men, the antibiotic ciprofloxacin raised it from about 5 hours to 8. d9-caffeine has not been studied with these drugs, so ask your pharmacist. L-theanine itself does not induce or inhibit these liver enzymes, according to Memorial Sloan Kettering.

How d9-caffeine changes the timeline

Regular caffeine has a half-life of about five hours in healthy adults, based on small pharmacology studies. d9-caffeine is caffeine with nine hydrogen atoms replaced by deuterium, which slows that breakdown. In a 2022 double-blind crossover study in 20 healthy adults, d9-caffeine was absorbed as quickly as regular caffeine, reached a 29 to 43 percent higher peak and produced four to five times the total exposure, with blood sampled for up to 48 hours. Both were well tolerated.

Longer exposure is the point of d9-caffeine, and it shapes side effects too. The effect arrives on the usual schedule but stays longer, so a dose that is fine at breakfast can reach into your night if taken in the afternoon, and any side effect may take longer to fade than it would with coffee. That is an inference: the study measured blood levels, not sleep or side effects. Take it in the morning and avoid late-day doses. Our guide to dose, timing and sleep lays out the schedule.

Side effects, likely causes and what to do

Who should check with a physician first

Skip it, 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, a sleep disorder or frequent migraines. Check with your physician or pharmacist if you take prescription medication, especially stimulant medication, blood pressure medication, sedatives or sleep medication. Seek care promptly for chest pain, fainting, a racing heartbeat that does not settle, or a sudden severe headache.

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 tally your day.

Frequently asked questions

What are the side effects of L-theanine and caffeine?

Mostly the caffeine ones: jitters, a faster heartbeat, stomach upset, headache and trouble sleeping. L-theanine itself has a short list of mild effects in trials.

Does L-theanine cancel out caffeine side effects?

Not reliably. In a 2008 study it blunted caffeine's rise in blood pressure but did not change jitteriness, while a small 2025 study in athletes found less anxiety and fewer reports of a fast heart rate with the pair.

Is it safe to take caffeine and L-theanine every day?

For most healthy adults, daily use within the label and the FDA's 400 mg caffeine reference point is reasonable. Stopping daily caffeine suddenly can cause withdrawal, and most L-theanine trials lasted eight weeks or less.

Can I take caffeine and L-theanine at night?

We do not recommend it. Caffeine taken six hours before bed disrupted sleep in a 2013 study, and d9-caffeine lasts longer than regular caffeine.

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.