L-Theanine

Found in tea leaves and consequently tea - not just green.

It is well absorbed from the intestine and can cross the blood brain barrier.

Potential benefits;

mental relaxation
alertness (especially if combined with caffeine as l-theanine takes around an hour to kick in, just when caffeine is wearing off)
improved sleep
improved selective attention (in those with mild cognitive impairment or high anxiety)
reduced stress (lowers cortisol levels)
may increase nitric oxide production

Forms/Types

Quality

As with all supplements - buyer beware. It's quite common for supplements to be contaminated with a variety of impurities, often in the form of heavy metals, and/or contain far more or less of the ingredients stated on the label. There's no simple rule of thumb to avoid this, such as avoiding cheap or imported products; expensive or well-known brands produced in-country may be just as bad. Ideally you should take some time to find products (if you can) that;

have been laboratory tested on behalf of the manufacturer by a reputable body or company that operates in your country or region (here's an example of a Peak Supps creatine supplement available on Amazon in the UK which has been tested by Eurofins: Creatine Monohydrate Powder 1Kg | Pure Micronised | Packaging May Vary - see pictures 3 & 4)
in the UK & EU, are manufactured in facilities that meet the minimum standards of good manufacturing practice (GMP) in their production processes (for medicines).¹² Facilities are inspected for compliance and manufacturers' product(s) must:
- be of consistent high quality
- be appropriate to their intended use
- meet the requirements of the marketing authorisation (MA) or product specification

Be wary of any manufacturers or brands making claims around these valid but irrelevant standards;

HACCP: this standard relates to food safety which indicates that the seller is claiming their supplement is a food; whilst meeting this standard is worthwhile, GMP (detailed above) would be a more appropriate standard
ISO22000: same as HACPP
ISO9001: this standard is focussed on quality control management systems in the context of managing a business or organisation, not directly on the the quality of the products that business produces (although it may have a beneficial impact)

Dosage - How Much?

400mg (this is far more than you would get in a cup of tea).

Deficiency - Too Little

Toxicity - Too Much

Side Effects

When to Avoid Use - Interactions/Contraindications

Random Facts

References

Metadata

Created: 2026-05-29

Last Updated: 2026-06-28