The NIH's official drug-injury database, LiverTox, now lists green tea extract as a well-established cause of serious liver injury.
Reviewers have logged more than 100 published cases of clearly apparent liver damage tied to the supplement, with some patients needing urgent liver transplants and a small number dying from acute liver failure.
The problem is not the beverage. It is the concentrated capsule sold for weight loss and antioxidant benefits.
LiverTox describes the reaction as idiosyncratic, meaning it strikes only certain people, and gives green tea extract a "likelihood score A", its top tier for confirmed liver-injury culprits.
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Why the pill is riskier than the cup
The active compound is EGCG, short for epigallocatechin-3-gallate, one of several catechin antioxidants found in green tea leaves.
In a brewed cup, EGCG comes in modest amounts alongside water and other tea compounds. In a capsule, it is stripped out and concentrated, and often taken on an empty stomach to boost absorption.
That matters because the European Food Safety Authority (EFSA) concluded in 2018 that daily EGCG intakes at or above 800 mg from supplements can raise liver enzymes in clinical trials, a signal of liver stress.
On the strength of that finding, the European Commission now requires supplement labels to cap daily servings below 800 mg of EGCG and to warn children under 18, pregnant women and nursing mothers. Brewed tea infusions are exempt.
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Many US green tea extract capsules provide several hundred milligrams of EGCG per serving.
Stacking multiple capsules a day, or combining a supplement with heavy tea drinking, can put a user near or above the EFSA threshold quickly.
The genetic clue
Most people who take green tea extract never have a liver problem, and researchers have been trying to work out why a minority do.
A study published in Hepatology in June 2021 by Jay H. Hoofnagle and colleagues, working with the US Drug-Induced Liver Injury Network, pointed to an immune-system explanation.
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The authors examined 40 cases of green tea-associated liver injury and found the tissue-type marker HLA-B*35:01 in 72% of them, compared with roughly 12 to 15% in people whose liver injury was caused by other drugs or supplements.
Patients ranged in age from 17 to 69, with a median of 40, and typically developed symptoms about 10 weeks after starting the product.
Most became jaundiced, meaning their skin and eyes turned yellow because the liver was struggling to clear bilirubin.
The pattern fits an idiosyncratic, immune-mediated reaction. EGCG appears to trigger T-cells in people who carry the marker, and their immune system then attacks liver cells.
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It also explains why most users tolerate the supplement while a few end up hospitalized.
What to look for on the label
Green tea extract is one of the most common ingredients in weight-management, "metabolism-boosting" and antioxidant capsules sold in the US.
Before starting a product, it is worth turning the bottle over and scanning the ingredient panel for "green tea extract", "Camellia sinensis extract" or "EGCG", and checking the milligrams per serving.
Warning signs of drug-induced liver injury include unusual fatigue, nausea, right-sided abdominal pain, dark urine and yellowing of the skin or eyes.
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Anyone taking a green tea extract supplement who notices these symptoms should stop the product and contact a doctor.
As LiverTox and other reviewers have consistently found, the same catechins that a person can drink safely as a daily cup of green tea behave differently once they are concentrated into pill form.
The EFSA labeling rules and daily cap on EGCG in supplements took full effect across the EU in June 2023.
In the US, no equivalent federal limit is in place, so the dose in a bottle of green tea extract can vary widely from brand to brand.
This article is made and published by Mie Hermansen, who may have used AI in the preparation.