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Green tea extract
Green tea extract — what the European monograph actually supports, what it is not for, the form and how long it is meant to be used, side effects, and the 3 medicine interactions documented for it.
This is a reference page, not a diagnosis and not a prescription. Herbal products are real pharmacology: they have effects, side effects and interactions with medicines. Tell your doctor and your pharmacist everything you take, including teas and supplements, and never stop or change a prescribed medicine because of anything you read here.
What it is
Camellia sinensis (L.) Kuntze — leaf; concentrated catechin extracts
Local names across the region this site serves: roheline tee (ET) · vihreä tee (FI) · зелёный чай (RU).
Grouped here under Kitchen spices with a safety story. Regulatory position: Drinking tea is a food; concentrated extracts are food supplements with an EFSA safety assessment.
What the evidence supports
The registered indication: Marketed for weight loss and metabolism. Neither claim survives review.
The gap between the drink and the capsule is the entire story here. Drinking green tea is not associated with liver harm and is a perfectly ordinary part of a diet. Concentrated green-tea extracts, sold for weight loss and metabolism, are a different exposure altogether: EFSA assessed them and concluded that catechin doses at or above 800 mg of EGCG a day taken as a food supplement have been shown in trials to raise liver transaminases — a marker of liver stress — and that a safe level for supplements could not be established from the available data. Green-tea extract is among the more frequently implicated botanicals in registries of herb-induced liver injury, and taking it on an empty stomach appears to raise absorption and risk. On the weight-loss claim, the effect measured in trials is too small to matter to anyone.
D — Popular claim unsupported, or safety dominates The better trials are negative, or the substance appears in the literature mainly for the harm it has caused.
What it is not for
Weight loss, and any liver purpose whatsoever — this is one of the botanicals most likely to injure a liver, not support one.
Form, strength and how long
Brewed leaf tea (a food), and concentrated catechin extracts in capsules, often combined with caffeine in "fat burner" products.
Duration. For extracts, the honest answer is that no safe long-term intake has been established.
No gram figure appears on this page on purpose. The plant part, the extraction solvent and the drug-to-extract ratio together define a preparation, and a number taken from one does not describe another. The reasoning is set out on the hub; the amount that applies to you is the one printed on the pack you are holding, and a pharmacist will read it with you.
Side effects
From tea, the effects of caffeine. From extracts, liver injury: dark urine, yellow eyes or skin, itching, unusual fatigue, pain under the right ribs. Stop immediately and seek medical care if those appear. Nausea and stomach upset are common, particularly fasted.
Who should avoid it
Avoid extracts with any liver condition, in pregnancy and while breastfeeding. Anyone taking a supplement with an unexplained rise in liver enzymes should stop it and tell their doctor what they were taking.
Interactions with medicines
| Medicine or substance | What is described |
|---|---|
| Anything else that stresses the liver | Stacking a green-tea extract with alcohol or with other substances that stress the liver is the pattern behind many of the reported cases. |
| Iron supplements and iron-rich meals | Tea polyphenols bind non-haem iron and measurably reduce how much iron is absorbed from a meal taken with tea. If you are being treated for iron deficiency, take iron and tea well apart. |
| Anticoagulants and antiplatelets (warfarin, DOACs, aspirin, clopidogrel) | Green tea leaf contains vitamin K, and large changes in intake have been reported to affect warfarin control. As with all the vitamin K foods, steady intake is what matters, not avoidance. |
The reverse view — start from the medicine rather than the plant — is in the medicine, food and alcohol interactions reference, and summarised for every herb on this site.
As a food
This one is also in the kitchen, so the nutrition figures are worth having next to the pharmacology: see the food data page for calories and nutrients per 100 g and per portion. Culinary amounts and medicinal preparations are different exposures, and the difference is usually the whole safety story.
Sources: European Food Safety Authority — scientific opinions on coumarin, green tea catechins and hydroxyanthracene derivatives · LiverTox: Clinical and Research Information on Drug-Induced Liver Injury (NIDDK, National Institutes of Health) · NIH National Center for Complementary and Integrative Health — herb fact sheets · checked 2026-09-06
Frequently asked questions
What is green tea extract used for?
Marketed for weight loss and metabolism. Neither claim survives review. The gap between the drink and the capsule is the entire story here. Drinking green tea is not associated with liver harm and is a perfectly ordinary part of a diet. Concentrated green-tea extracts, sold for weight loss and metabolism, are a different exposure altogether: EFSA assessed them and concluded that catechin doses at or above 800 mg of EGCG a day taken as a food supplement have been shown in trials to raise liver transaminases — a marker of liver stress — and that a safe level for supplements could not be established from the available data. Green-tea extract is among the more frequently implicated botanicals in registries of herb-induced liver injury, and taking it on an empty stomach appears to raise absorption and risk. On the weight-loss claim, the effect measured in trials is too small to matter to anyone.
Does green tea extract work?
Graded D here: The better trials are negative, or the substance appears in the literature mainly for the harm it has caused. Weight loss, and any liver purpose whatsoever — this is one of the botanicals most likely to injure a liver, not support one.
How much green tea extract should I take?
This reference does not print gram figures, because they do not transfer between preparations of different extract strength. What can be said honestly is the form and the duration: Brewed leaf tea (a food), and concentrated catechin extracts in capsules, often combined with caffeine in "fat burner" products. For extracts, the honest answer is that no safe long-term intake has been established.
What are the side effects of green tea extract?
From tea, the effects of caffeine. From extracts, liver injury: dark urine, yellow eyes or skin, itching, unusual fatigue, pain under the right ribs. Stop immediately and seek medical care if those appear. Nausea and stomach upset are common, particularly fasted.
Does green tea extract interact with medicines?
Yes — documented for Liver load, Iron, Anticoagulants, warfarin. Each one is described on this page, with a link to the medicine-side reference where one exists.
Others in kitchen spices with a safety story
Cinnamon
Nothing that survives a systematic review. It is a spice with a real safety figure attached.
Garlic
Relief of symptoms of mild circulatory complaints, in traditional-use preparations. Not the treatment of cardiovascular disease.
Nutmeg
Nothing medicinal. It is on this list because it is the kitchen spice that poisons people.
Chilli and cayenne (capsaicin)
Topical relief of muscle and joint pain in low-strength preparations; high-concentration capsaicin patches are licensed medicines for peripheral neuropathic pain.
Keep the whole list in one place
TempoLife logs supplements alongside food, sleep and how you actually felt — so the answer to "what are you taking?" is a screen, not a guess.