Green tea, white tea, and black tea are made from dried leaves of Camellia sinensis, a perennial evergreen shrub. Up to 40% of the dry weight of tea leaves is accounted for by antioxidants called catechins; epigallocatechin gallate (EGCG) is the most abundant catechin found in green tea. EGCG could theoretically promote brain health, but no studies have evaluated whether it can protect from cognitive decline or dementia. EGCG supplements are considered safe for most people but high doses may affect liver function.
Long-term studies are lacking and no studies have evaluated whether EGCG may slow cognitive decline or prevent dementia. Our search identified:
• 3 double-blind randomized clinical trials (1 in Down syndrome patients, 2 in healthy adults)
• 1 pilot clinical study in people with Down syndrome
• Numerous preclinical studies
Two small trials suggest that EGCG treatment may very modestly improve memory functions, inhibitory control, and adaptive behavior in people with Down syndrome, though this protective effect was evident in only 3 out of 24 cognitive tests . In healthy adults, a single dose of EGCG has had little influence on cognition. One double-blind randomized controlled trial reported that a single treatment did not affect cognitive performance or mood . A second trial reported increased calmness and reduced stress . EGCG also increased brain waves, which have been associated with relaxation, focused attention, and quiet wakefulness . Long-term studies in healthy people are needed to evaluate the effects of EGCG on cognitive health.
Preclinical studies have found a wide range of actions of EGCG, including chelating metals, reducing inflammation, scavenging free radicals, improving mitochondrial function, and preventing death of brain cells . EGCG may also inhibit enzymes that break down the neurotransmitter acetylcholine , which is important for memory retrieval. Other preclinical studies found that EGCG treatment improved function or reduced damage in the brain [9-11], but these effects have not been confirmed in human trials.
No studies have reported whether EGCG alone is beneficial to patients with dementia. While some benefits have been observed in preclinical studies of Alzheimer's disease, including improved cognitive function  and reduction of pathological markers of Alzheimer's , these effects have yet to be confirmed in people with dementia.
EGCG supplements are considered safe for most people when taken at commonly used doses (300–400 mg/day) , but high doses (800–1600 mg of EGCG per day) may negatively affect liver function . Most safety data on long-term EGCG intake come from large meta-analyses of tea consumption, which have reported that side effects are mild  and can include nausea and upset stomach .
Three drugs—warfarin (also known as Coumadin™ and Jantoven™), anisindione (or Miradon™), and dicumarol—are known to interact with green tea that contains EGCG, but the interactions are minor . For more information on green tea, please review our separate report.
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.
A single cup (200 ml) of green or white tea contains 25–200 mg of EGCG [19-21]. Black tea contains much lower levels of EGCG (~20 mg) as it is converted during the oxidation process to thearubigin, a different type of polyphenol . EGCG and other tea catechins are also available as dietary supplements, often in doses of 200–300 mg per serving, which are comparable to doses deemed safe in clinical trials . Higher doses can pose health risks, for example to liver function. Clinical trials examining the effects of EGCG on cognitive function have used doses ranging from 9 to 300 mg/day .
Download full scientific report
An analysis of commercially available green tea supplements at Labdoor
A review of test results on green tea supplements and tea at ConsumerLab.com
Quality Control of Sources: 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.