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.
Several 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:
• 1 meta-analysis based on 10 randomized controlled trials testing acute effects of L-theanine alone or in combination with caffeine in healthy adults
• 6 randomized controlled trials (3 testing acute effects, 2 examining brain activity, and 1 with green tea extract in people with mild cognitive impairment)
• 1 open-label study in people with major depressive disorder
• 1 review
• Numerous preclinical studies
The long-term effects of L-theanine on cognitive health are unknown, but a few short-term human studies have shown small benefits. 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 . The patients who had more severe impairment at the start of the trial appeared more likely to benefit from treatment, but these results have not yet been replicated.
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 . L-theanine may interact with caffeine, improving attention and ability to ignore distractions, and together, enhance performance on cognitively demanding tasks . In a small randomized controlled trial of healthy adults, L-theanine alone did not affect attentional focus . Other studies have reported mixed effects, with L-theanine decreasing the beneficial effects of caffeine on cognition and mood .
Preclinical studies have reported that L-theanine may reduce brain cell death  and oxidative damage , and increase levels of protective chemicals , 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 , L-theanine has not been clinically studied in dementia patients.
Large meta-analyses of long-term tea consumption (containing L-theanine) have found that side effects are mild  but these results on tea may not apply to supplements. In a short randomized controlled trial, an L-theanine supplement (400 mg/day) was well-tolerated with no significant adverse events . Larger, long-term studies are needed. For more information on green tea, please review our separate report.
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 . Because of its possible effects on blood pressure , 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 using 200 mg/day .
An analysis of commercially available products labeled as L-theanine.
United States Pharmacopeial Convention (USP) and FDA Information on Dietary Supplements offer information on the quality of specific supplements and can assist in finding a trusted brand.