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L-theanine and attention switching: grade B

Moderate evidence. Direction: Benefit.

B ModerateIndustry-fundedShort-term only

Educational guidance, not medical advice

Why this grade

Rubric condition met: >=2 RCTs with mostly consistent results (B); A held back because the supporting trials are small and mostly led by Unilever researchers (citation-rules 9 and 10). Context: L-theanine taken with caffeine.

Two meta-analyses (a 2014 review of 11 studies and a 2025 review co-funded by Lipton) found that L-theanine with caffeine improved attention-switching accuracy within 2 hours compared with placebo. Three small RCTs of 97-100 mg L-theanine with 40-50 mg caffeine agree, but all three were led by Unilever researchers. The 2014 moderator analysis suggests caffeine dose drives much of the effect, so the benefit cannot be credited to L-theanine alone. Effects are acute only.

How we grade evidence

Studies this grade rests on

EffectMixedNo effectSize shows participants (n)200520102015202020252030Meta-analysisRCTCrossover RCTGiesbrecht 2010, 2010: RCT, n = 44, result not recordedOwen 2008, 2008: crossover RCT, n = 27, result not recordedEin 2010, 2010: crossover RCT, n not recorded, result not recordedCamfield 2014, 2014: meta-analysis, n not recorded, result not recordedPayne 2025, 2025: meta-analysis, n not recorded, result not recorded
each study this grade rests on, by publication year and design. Solid: effect versus the comparator; half-filled: mixed; hollow: no effect; dashed: result not recorded yet. Larger marks had more participants.
Show the numbers
StudyYearDesignParticipantsResult
Owen 20082008crossover RCT27result not recorded
Ein 20102010crossover RCTnot recordedresult not recorded
Giesbrecht 20102010RCT44result not recorded
Camfield 20142014meta-analysisnot recordedresult not recorded
Payne 20252025meta-analysisnot recordedresult not recorded

The combined effects of L-theanine and caffeine on cognitive performance and mood.

Owen GN, Parnell H, De Bruin EA. The combined effects of L-theanine and caffeine on cognitive performance and mood Nutr Neurosci, 2008. PMID 18681988

Result not recorded
Design
Randomized placebo-controlled crossover trial (acute)
Sample
n = 27
Population
Healthy volunteers
Funding
not stated in PubMed record

L-theanine and caffeine improve task switching but not intersensory attention or subjective alertness.

Einöther SJ, Martens VE, Rycroft JA. L-theanine and caffeine improve task switching but not intersensory attention or subjective alertness Appetite, 2010. PMID 20079786

Result not recorded
Design
Randomized, placebo-controlled, double-blind crossover trial (acute)
Population
Adults (n not stated in abstract)
Funding
not stated in PubMed record

The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness.

Giesbrecht T, Rycroft JA, Rowson MJ. The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness Nutr Neurosci, 2010. PMID 21040626

Result not recorded
Design
Randomized placebo-controlled trial (acute)
Sample
n = 44
Population
Young adults
Funding
not stated in PubMed record

Acute effects of tea constituents L-theanine, caffeine, and epigallocatechin gallate on cognitive function and mood: a systematic review and meta-analysis.

Camfield DA, Stough C, Farrimond J. Acute effects of tea constituents L-theanine, caffeine, and epigallocatechin gallate on cognitive function and mood: a systematic review and meta-analysis Nutr Rev, 2014. PMID 24946991

Result not recorded
Design
Systematic review and meta-analysis of 11 randomized placebo-controlled studies
Population
Healthy adults
Funding
not stated

Effects of Tea (Camellia sinensis) or its Bioactive Compounds l-Theanine or l-Theanine plus Caffeine on Cognition, Sleep, and Mood in Healthy Participants: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.

Payne ER, Aceves-Martins M, Dubost J. Effects of Tea (Camellia sinensis) or its Bioactive Compounds l-Theanine or l-Theanine plus Caffeine on Cognition, Sleep, and Mood in Healthy Participants: A Systematic Review and Meta-Analysis of Randomized Controlled Trials Nutr Rev, 2025. PMID 40314930

Result not recorded
Design
Systematic review (50 RCTs) and meta-analysis (15 eligible)
Population
Healthy participants
Funding
Analytical Services Division RI-B5-06, Lipton Teas and Infusions, Scottish Government Rural and Environment Science, Scottish Government Rural and Environment Science and Analytical Services Division, Scottish Government Rural and Environment Science and Analytical Services Division RI-B5-06

Search and history

Search (): theanine[ti] AND caffeine[ti] AND (attention[tiab] OR cognitive[tiab] OR "task switching"[tiab])

  • Graded B (proposed): Proposed in the monograph draft
  • B to B (approved): Editor sign-off November 13, 2026: grade matches the editor-approved final page