Green tea is prepared from dried leaves of Camellia sinensis, a perennial evergreen shrub. It contains several compounds that are possibly beneficial to brain health, including caffeine, catechins (polyphenols like EGCG), and L-theanine (an amino acid derivative). Numerous observational studies suggest that greater green tea consumption is associated with better cognitive function and lower dementia risk. Studies have shown that drinking green tea is safe at moderate and regular amounts.
Observational studies have found that greater green tea consumption is associated with lower risks of dementia and cognitive decline. Evidence from clinical trials is less consistent, possibly due to variations in the forms of green tea/extract tested, the duration of the studies, and the cognitive outcomes tested. Our search identified:
Three meta-analyses, each including numerous observational studies, have reported that greater green tea consumption is associated with a 33-47% lower risk of cognitive impairment and a 25-29% lower risk of dementia [1; 2; 3]. Two of the meta-analyses reported that the association between green tea consumption and better cognitive health was observed in Asian populations but not in European populations, which may reflect the relative lack of data from non-Asian populations [1; 2].
Three double-blind randomized controlled trials have evaluated the effects of green tea/extract on cognitive functions. In one trial in 99 older adults with subjective cognitive decline or mild cognitive impairment, matcha green tea (2 grams) taken daily for 12 months improved social acuity score compared to placebo, but not global cognition or other cognitive functions including attention, cognitive flexibility, and executive attention [4]. A trial in 91 patients with mild cognitive impairment reported that the combination of green tea extract and L-theanine for 16 weeks resulted in significant improvements in memory and attention, particularly in patients who had relatively severe baseline impairment [5]. The third trial examined the acute effects of a drink containing 27.5 g of green tea extract and reported that the drink increased brain connectivity associated with working memory, and the degree of connectivity correlated with the magnitude of improvement in working memory [6]. Green tea contains several compounds that may have beneficial properties, including caffeine, L-theanine, and green tea catechins (e.g., EGCG) [7; 8].
No studies have reported whether green tea can improve cognition or slow decline in people with dementia. In a clinical trial, markers of oxidative stress were decreased in Alzheimer’s patients who consumed a beverage that included green tea extracts for eight months [9]. However, it is unknown whether this beverage helps patients.
Multiple meta-analyses have reported that green tea consumption is safe at moderate and regular amounts (3 to 5 cups per day, up to 1200 ml/day) and side effects are mild [10; 11; 12]. It is worth noting that green tea supplements such as green tea extract at high doses may result in some adverse effects including gastrointestinal issues, elevation of liver enzymes, and sleep problems [13]. Three drugs are known to interact with green tea: warfarin (also known as Coumadin™ and Jantoven™), anisindione (or Miradon™), and dicumarol [14]. Green tea consumption may reduce the levels of folic acid in the body and interfere with iron absorption [14]. Because green tea contains caffeine, pregnant women and people with cardiovascular problems or other health risks should consult their physician or healthcare provider about consuming green tea. Caffeine in green tea can also interact with some medications [15].
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.
Green tea is available loose or in tea bags. A typical cup of brewed green tea (240 mL) contains 30–60 mg of caffeine, 8–25 mg of L-theanine, and 25–200 mg of catechins [16; 17; 18]. Additionally, green tea supplements (e.g., green tea extract, catechins, L-theanine) are available in the forms of pills, capsules, liquid, and powder.
Full scientific report (PDF) on Cognitive Vitality Reports
Information on green tea from the National Center for Complementary and Integrative Health
Additional information on caffeine doses in common drinks from the Mayo Clinic
An analysis of commercially available green tea supplements at Labdoor
Check for drug-drug and drug-supplement interactions on Drugs.com