- Flavonoids Linus Pauling Institute
- Tea Linus Pauling Institute
- Green tea WebMD
- Scalon C et al, 2013, Flavonoids for treating venous leg ulcers Cochrane
- Hartley R et al, 2013, Green and black tea to prevent cardiovascular disease Cochrane
- Jurgens TM et al, 2012, Green tea for weight loss and weight maintenance in overweight or obese adults Cochrane
- Boehm K et al, 2009, Green tea for the prevention of cancer Cochrane
- Gupta AK et al, 2015, Sinecatechins 10% ointment: a green tea extract for the treatment of external genital warts PubMed
- Veregen ointment Food and Drug Administration
- Tzellos TM et al, 2011, Efficacy, safety and tolerability of green tea catechins in the treatment of external anogenital warts: a systematic review and meta-analysis PubMed
- Tatti S et al, 2008, Sinecatechins, a defined green tea extract, in the treatment of external anogenital warts: a randomized controlled trial PubMed
- Gren tea National Center for Complementary and Integrative Health
- Mascitelli L et al, 2010, Inhibition of iron absorption by polyphenols as an anti-cancer mechanism QJM
Flavanols (Catechins) in Green Tea Extracts
What are flavanols?
Flavanols are a class of flavonoids. They are nonessential nutrients found mainly in tea. They include catechins (epicatechin, epicatechin gallate, epigallocatechin, epigallocatechin gallate), theaflavins, thearubigins and proanthocyanidins .
Teas High in Flavanols
- Green tea extracts are high in catechins and black tea is high in flavanols theaflavins and thearubigins . Tea is also high in flavonols kaempferol, quercetin, and myricitin .
- Cocoa and dark chocolate are also high in flavanols .
Health Benefits of Green Tea
Green tea extracts are LIKELY EFFECTIVE in:
- External genital warts (as a prescribed ointment) [3,8,10,11]; FDA-approved 
- Mental alertness (due to caffeine) [3,12]
There is INSUFFICIENT EVIDENCE  about the effectiveness of green tea or its extracts in the prevention or treatment of acne, allergies, amyloidosis, cancer (bladder, breast, colorectal, gastric, lung, mouth, endometrial, ovarian, prostate) , cervical dysplasia, cold and flu, coronary heart disease or other cardiovascular disease , dental caries , diabetes, fertility in women, high blood pressure, high total and LDL cholesterol , kidney stones , leukemia, low blood pressure, metabolic syndrome, osteoporosis, Parkinson’s disease, periodontal disease (gum disease), pneumonia, stroke, upper respiratory tract infection, venous leg ulcers , wrinkled skin or in reducing stress, promoting weight loss [2,6] or improving exercise performance.
Tea Extracts Safety: Side Effects, Toxicity
Green tea is LIKELY SAFE for most adults when drunk in moderate amounts . Drinking more than 5 cups of green tea per day is POSSIBLY UNSAFE .
Decaffeinated tea extracts may cause mild nausea, stomach upset, dizziness, muscle pain. Other reported side effects of tea extracts, such as diarrhea, restlessness, insomnia, tremor, confusion, may be caused by caffeine . Tea may reduce the absorption of iron from plant foods and therefore worsen anemia [2,3,6,13].
Green tea may trigger irregular heartbeats in sensitive people, make anxiety, glaucoma, irritable bowel syndrome or osteoporosis worse, increase the risk of bleeding and increase blood pressure .
Not enough is known about the safety of tea extracts during pregnancy and breastfeeding, so women in these periods should avoid them .
Green tea may be toxic for liver .
Consumption of large amounts of green tea may reduce the effect of adenosine, lithium, pentobarbital and blood thinners aspirin, clopidogrel, diclofenac, ibuprofen, naproxen, dalteparin, enoxaparin, heparin and warfarin [2,3].
Green tea may increase the effect of amphetamines, cocaine, ephedrine, nicotine, phenylpropanolamine and certain antidepressants (MAO inhibitors) .
Certain antibiotics (ciprofloxacin, enoxacin, norfloxacin, grepafloxacin), birth control pills, cimetidine, clozapine, disulfiram, estrogens, fluvoxaine, riluzole, theophylline and verapamil may enhance side effects of caffeine from green tea, such as jitteriness, headache, increased heart rate .