Amino acid
L-Theanine
γ-glutamylethylamide
Last reviewed
What the research found
A four-week randomised, double-blind crossover trial gave 30 healthy adults 200 mg/day of L-theanine or placebo. Self-rated depression, trait anxiety and sleep-quality scores all fell during the theanine period (p = 0.019, 0.006 and 0.013). Worth knowing when you read it: two of the authors work for Taiyo Kagaku, the company that makes Suntheanine® and supplied the tablets.
"Our findings suggest that L-theanine has the potential to promote mental health in the general population with stress-related ailments and cognitive impairments."
Source: Hidese et al., Nutrients, 2019
Our pick, and why
Choosing a l-theanine: how the options compare
Doctor's Best is our pick: it's the only one here that explicitly names Suntheanine® on the label — the branded form with its own published trial record — at 150 mg, within the studied range. NOW Foods delivers 200 mg and is a highly reputable manufacturer, but neither the label nor the Amazon listing names the source of its L-theanine, so we cannot confirm it matches the trialed form. Jarrow is in the same position. Both are likely fine in practice, but on a site that takes evidence transparency seriously we can only highlight the one that states its pedigree. Honest caveat: if dose matters more to you than brand pedigree, 200 mg from NOW or Jarrow is a reasonable, lower-cost choice.
How we judge these
Suntheanine® branded form stated on label, dose 100–200 mg, no unstated caffeine.
| Product | Branded form | Dose / serving | Added caffeine | Other actives | Meets it | Link |
|---|---|---|---|---|---|---|
| Our pick Doctor's Best L-Theanine with Suntheanine® | Suntheanine® | 150 mg | No | None | ✓ | Amazon → |
| NOW Foods L-Theanine 200 mg | Not stated | 200 mg | No | Inositol 100 mg | ✓ | Amazon → |
| Jarrow Formulas Theanine 200 | Not stated | 200 mg | No | None | ✓ | Amazon → |
- Doctor's Best L-Theanine with Suntheanine®Our pick
- Branded form
- Suntheanine®
- Dose / serving
- 150 mg
- Added caffeine
- No
- Other actives
- None
- Meets our criterion
- ✓
- NOW Foods L-Theanine 200 mg
- Branded form
- Not stated
- Dose / serving
- 200 mg
- Added caffeine
- No
- Other actives
- Inositol 100 mg
- Meets our criterion
- ✓
- Jarrow Formulas Theanine 200
- Branded form
- Not stated
- Dose / serving
- 200 mg
- Added caffeine
- No
- Other actives
- None
- Meets our criterion
- ✓
Specs verified against live listings at time of writing and can change — confirm on the product page. Links are affiliate links; details.
L-Theanine at a glance
These six measurements are recorded the same way for every supplement on this site, so you can compare across pages. How we measure.
- Dose used in trials
- 200 mg/day for four weeks in the stress and sleep trial. Single-dose studies of acute calming use 100 to 200 mg.
- Dose in our pick
- 150 mg per capsule, below the 200 mg used in the four-week trial
- Standardised form
- Suntheanine®, named on the label
- Third-party tested
- Not stated on the label
- Other actives
- None
- Reported onset
- About 30 to 60 minutes for acute calming. The sleep and anxiety changes above were measured over 28 days.
L-theanine is the reason a cup of green tea feels calmer than a cup of coffee, despite both containing caffeine. It increases alpha-wave activity — the brain rhythm associated with relaxed, wakeful states — and unlike most calming compounds it does this without making you sleepy.
That “relaxed but alert” quality makes it one of the few stress-support ingredients that’s equally at home before a stressful meeting and before bed.
What to look for
- Plain L-theanine (Suntheanine is a well-known branded form)
- 100–200 mg per serving
- Optional caffeine + theanine combos if focus is your main goal
Common questions about l-theanine
How much L-theanine do the studies actually use?
Single-dose studies of acute calming generally use 100 to 200 mg. The four-week trial that tracked anxiety, sleep and depression scores used 200 mg a day (Hidese et al., 2019). Our pick delivers 150 mg per capsule, which sits inside the acute range but below the four-week dose, so two capsules would overshoot it slightly and one falls short. Neither is a problem at this safety profile. We cover the reasoning in more depth in the L-theanine dosage guide.
Is Suntheanine® actually better than generic L-theanine?
Chemically they may well be equivalent. The difference is what you can verify. Suntheanine® is an enzymatically produced form made by Taiyo Kagaku, and it is the material used in most published trials, including the 2019 four-week study. A label reading only "L-theanine" does not tell you what is in the capsule or how it was made. That is the whole basis for our pick, and it is worth being clear that it is an argument about traceability rather than a claim that generic theanine does not work.
How long does L-theanine take to work?
People generally report the calming effect within 30 to 60 minutes of a single dose, which is why it gets taken before a stressful meeting or alongside coffee. The measured changes in sleep quality and trait anxiety in the 2019 trial came after four weeks of daily use, so the two timescales answer different questions. There is a longer breakdown in how long L-theanine takes to work.
Can you take L-theanine with caffeine?
Yes, and that combination is the reason a lot of people take it. Theanine is what makes green tea feel calmer than coffee despite both containing caffeine, and it is commonly used to blunt the jittery edge of a large dose. Check the label before stacking, because some theanine products already include caffeine. Ours does not. See the caffeine and theanine stack for typical ratios.
Does L-theanine make you drowsy?
Not usually. It is one of the few calming compounds that does not act as a sedative, which is why it gets described as relaxed alertness rather than sleepiness. The 2019 trial found improvements in verbal fluency and executive function alongside the anxiety and sleep changes, so cognitive performance held up. If you want it specifically for sleep, see L-theanine for sleep.