L-theanine is an amino acid structurally similar to glutamate and GABA, two neurotransmitters? important for brain function. It is contained in green, black, and oolong teas, which are all derived from Camellia sinensis, a perennial evergreen shrub. L-theanine is traditionally used to promote relaxation without sedative effects. Research suggests it is safe and has positive effects on cognitive function when combined with caffeine, but the effects of L-theanine alone appear to be modest and short-term. No studies have tested whether it can prevent dementia or cognitive decline.
Numerous clinical trials have examined the effects of L-theanine, alone or in combination with caffeine or green tea, on brain health. No human studies have tested it for the prevention of dementia or cognitive decline. Our search identified:
• 3 systematic reviews or meta-analyses of clinical trials testing L-theanine • 15 randomized controlled trials • 1 open-label study in people with major depressive disorder • 1 review • Numerous preclinical studies
Short-term studies of L-theanine have reported some positive effects on attention and relaxation [1; 2; 3; 4], but studies of chronic intake have not shown improved cognitive function [5; 6; 7]. For example, in a randomized controlled trial of 69 people aged 50-69 years old, L-theanine treatment for 12 weeks did not significantly affect cognitive functions [5]. In a double-blind randomized controlled trial of 30 middle-aged people, L-theanine treatment for four weeks improved a few cognitive functions compared to baseline, but no significant differences were found for any of the 12 cognitive outcomes when compared to placebo [6]. In a randomized controlled trial in patients with mild cognitive impairment, L-theanine taken with green tea extract for 16 weeks had no significant effect on memory and attention, though a trend for an improvement in memory was seen midway through the trial [7].
Combinations of L-theanine and caffeine have been reported to acutely improve attention and alertness in small clinical trials, but the positive effects have been primarily attributed to caffeine [8]. L-theanine may interact with caffeine, improving attention and ability to ignore distractions, and together, enhance performance on cognitively demanding tasks [9]. In a small randomized controlled trial of healthy adults, L-theanine alone did not affect attentional focus [10]. Other studies have reported mixed effects, with L-theanine decreasing the beneficial effects of caffeine on cognition and mood [11].
Preclinical studies have reported that L-theanine may reduce brain cell death [12] and oxidative damage [13], and increase levels of protective chemicals [14], but these effects have not been confirmed in humans.
It is currently unknown whether L-theanine can improve cognition or slow cognitive decline in people with dementia. While some benefits have been observed in preclinical studies of Alzheimer’s disease [12], L-theanine has not been clinically studied in dementia patients.
In a meta-analysis of 18 randomized controlled trials, L-theanine treatment was generally well tolerated with headache, migraine, and gastrointestinal disorders being the most frequently reported symptoms [15].
The interactions between L-theanine and other drugs have not been well-studied. Clinical data suggest that L-theanine lessens the blood pressure increase caused by caffeine [16; 17]. Because of its possible effects on blood pressure [16; 18], it may be dangerous to use in combination with blood pressure medications.
NOTE: This is not a comprehensive safety evaluation or complete list of potentially harmful drug interactions. It is important to discuss safety issues with your physician before taking any new supplement or medication.
L-theanine supplements are available in both capsule and powder forms. A single cup (200 ml) of tea can contain 5 to 85 mg of L-theanine depending on the type, quality, and preparation of tea. Clinical trials examining the effects of L-theanine on cognitive function have used doses ranging from 12–250 mg/day, with the majority of studies testing 200 mg/day [4; 8; 19].
Full scientific report (PDF) on Cognitive Vitality Reports
An analysis of commercially available products labeled as L-theanine