Green Tea: What Research Shows About the Tea, Safety and Cautions

Check before you drink: green tea has caffeine, so pregnancy needs a limit (NHS: 200 mg a day in total) [11]; tea can lower how much iron you absorb from a meal [7]; and one case report describes warfarin becoming less effective after very large daily amounts of green tea [8]. Green tea is a "true tea" from the leaves of the tea plant. This page is about the cup of tea itself. Capsules and extracts (EGCG) are a different product with a liver-injury warning; they are covered on the green tea herb page.

What green tea is

Green tea comes from the leaves of Camellia sinensis. The leaves are steamed or pan fried and dried but not fermented, which is what separates green tea from black tea (fully fermented) and oolong (partly fermented). [1] It contains caffeine, and most of its polyphenols are catechins; the main one is EGCG. [1][2] Its taste is usually described as fresh and grassy, sometimes a little bitter or nutty; we found no independent source that measures taste, so treat that as a general description. For the powdered, whole-leaf form see Matcha. For the comparison with black, white and oolong see green, black, white and oolong tea.

Taste, caffeine and how to brew

What research found about the tea itself

All of the Cochrane reviews mix drinks with extracts, so they cannot tell you what a cup of tea does. Most studies are short (weeks to a few months), small and done in a few countries, so they say little about drinking tea for years. Observational studies of tea drinkers cannot rule out that tea drinkers differ in other ways. [5][1]

Check before you drink

• Pregnancy and breastfeeding. The NHS says no more than 200 mg of caffeine a day in pregnancy, and gives about 75 mg for a cup of black or green tea. [11] Health Canada gives a limit of 300 mg a day for people who are pregnant, planning to become pregnant or breastfeeding. [10] NCCIH says to keep caffeine to moderate levels and ask your provider. [1] LactMed says very high maternal caffeine has been linked with fussy, jittery infants with poor sleep, and finds no reports of anemia in breastfed infants from mothers drinking tea. [13] See pregnancy teas to ask about and breastfeeding.
• Babies and young children. The FDA quotes the Dietary Guidelines: avoid caffeinated drinks for children younger than 2. [9] LactMed says tea given directly to infants can interfere with iron absorption and cause anemia. [13] See herbal tea for babies and children.
• Medicines. NCCIH says high doses of green tea lowered blood levels of the beta-blocker nadolol, and that green tea may interact with other medicines (it also names raloxifene and atorvastatin; the atorvastatin finding is for extract). Talk to a pharmacist if you take any medicine. [1] Warfarin: one 1999 case report describes a man on warfarin whose blood-clotting test (INR) fell from 3.79 to 1.37 after about a week of drinking half a gallon to a gallon of green tea a day, and returned to 2.55 after he stopped; the authors suggested vitamin K in the tea. [8] It is a single case and cannot prove cause, and it involved very large amounts. Ask your prescriber before changing your tea habits. See herbal tea and medicine interactions.
• Liver. NCCIH reports no safety concerns for green tea as a drink in adults, and says liver injury, though uncommon, has been reported mainly with extracts. [1] EFSA (2018) judged green tea infusions prepared the traditional way to be generally safe at the intakes seen in Europe, but noted rare liver injury cases after infusions, "most probably" an unusual individual reaction. [2] LiverTox says extract, and more rarely very large amounts of tea, have been linked to liver injury. [3] See liver and kidney cautions.
• Iron. A radio-iron study in adults found that black tea cut iron absorption from a bread meal by 79 to 94 percent, and that herbal teas also reduced it (chamomile 47 percent, peppermint 84 percent). Green tea was not among the drinks tested. [7] A 2024 case report describes severe iron-deficiency anemia in a woman after about three months of moderate green tea, which improved after she changed her tea habits. [16] If you have low iron, ask a clinician; drinking tea between meals rather than with them is a common suggestion, but the sources we read do not test that. See acid reflux, teeth and iron absorption.
• Caffeine. The FDA cites 400 mg a day as an amount not generally linked with harm for most adults, with wide differences in sensitivity. [9] Decaffeinated tea still contains some caffeine. [9]
• Very hot drinks. See the hot-drink row above and hot tea and esophageal cancer risk. [12]
• Lead, other metals, pesticides, fluoride. Proper evidence not found for a measured figure for green tea in the sources we read. Not described is not proof of none. See heavy metals in herbal tea.
• Allergy and kidney stones. Not described in the sources we read for green tea.

Tea, capsule or extract?

A cup of green tea is a weak water extract of the leaf. A capsule or "standardized extract" packs catechins into a small pill, and the liver warnings and several trial results belong to that form. EFSA put the liver-test effect at 800 mg of EGCG a day or more as a supplement, while it estimated that average EGCG from infusions in Europe is 90 to 300 mg a day. [2] Do not carry a result from one form to the other. See the green tea herb page, green tea vs matcha vs extract and tea vs tincture vs capsule.

Buying and storing

• Check the label for the plant name (Camellia sinensis) and whether it is a blend; blends can add herbs or flavors. See reading an herbal tea or tincture label.
• Loose leaf or bags: Proper evidence not found in the sources we read that one is safer.
• Storage and shelf life: see storing herbal tea and how long it lasts.

Related pages

• Green tea (herb page: supplements and extracts)
• Matcha
• Green, black, white and oolong tea
• Black tea
• Herbal tea vs true tea
• Caffeine in foods and drinks: official limits