Green tea extract side effects

The most consistently documented side effect is liver-related: a 2018 EFSA scientific opinion found EGCG doses at or above 800 mg a day raised liver enzymes in clinical trials, and rare case reports describe liver injury at doses both above and below that line. Nausea and caffeine-related effects — jitteriness, a faster heartbeat, trouble sleeping — are the other most commonly reported issues, and both depend heavily on dose and on the specific product's caffeine content.

Liver enzymes and liver injury

This is the side effect with the most regulatory attention. A 2018 EFSA scientific opinion reviewing the full published evidence concluded that interventional clinical trials showed EGCG intake at or above 800 mg a day from a food supplement produced a statistically significant increase in serum transaminases — liver enzymes used as a marker of liver stress — compared with control groups. The same opinion noted that rare cases of liver injury have also followed ordinary green tea infusion consumption, which the panel attributed most likely to an idiosyncratic (individual, unpredictable) reaction rather than dose alone (PMID 32625874).

A separate, more detailed 2020 review by a United States Pharmacopeia expert panel — including FDA liaisons and members of the NIH Drug-Induced Liver Injury Network — looked specifically at whether contaminants (solvent residues, pesticide residues, pyrrolizidine alkaloids, elemental impurities) might explain these cases, and found no evidence that they do. The injury pattern instead points to the catechins themselves, showing a hepatocellular pattern in toxicology studies. Published case reports span EGCG intakes from about 140 mg to 1,000 mg a day — meaning some people reported problems at doses well below EFSA's 800 mg threshold, while most people taking considerably more never report one. The review attributed part of this gap to genetic differences in individual susceptibility, and part to dosing condition: taking green tea extract in repeated bolus doses while fasting was found to substantially raise how much EGCG reaches the bloodstream, which the panel linked to the hepatotoxicity risk (PMID 32140423). See when is the best time to take green tea extract? for what that means in practice, and who should not take green tea extract? for who's advised to be more cautious.

Nausea and digestive upset

A 2005 crossover trial in 30 healthy adults tested single doses of 400, 800, and 1,200 mg EGCG, taken either fasting or with food. All three doses were generally well tolerated, and mild, transient nausea was the main side effect reported — occurring most often at the highest dose (1,200 mg) taken on an empty stomach (PMID 15958649). That pattern — nausea more likely at higher doses, and more likely fasting than fed — lines up with the same dosing-condition findings behind the liver-safety data above.

Caffeine-related effects

Unless a product specifically states it's decaffeinated, it likely carries some amount of the tea leaf's natural caffeine, and that amount varies substantially by brand. A 2007 USDA laboratory analysis of 53 commercial caffeine-containing supplements — a group that included products built around green tea extract, guarana, yerba mate, and kola nut — found actual per-day caffeine content ranging from 1 mg to 829 mg across products (PMID 17676317). Jitteriness, a faster heartbeat, and trouble sleeping are effects commonly associated with caffeine generally; no trial we reviewed measured these specific outcomes for a green tea extract product on its own, so how noticeable they are depends on a given product's actual caffeine content and an individual's caffeine sensitivity, not a fixed rate across all green tea extract supplements.

Questions people ask

Can green tea extract cause liver damage?

Trial data has linked EGCG doses of 800 mg a day or more to a statistically significant rise in liver enzymes, and rare published case reports describe liver injury across a wide dose range (140-1,000 mg EGCG/day). See who should not take green tea extract? for who's advised against it or to check with a doctor first.

Can green tea extract make you dizzy?

No trial we reviewed specifically measured dizziness as an outcome. It's a commonly reported effect of caffeine in general use, so a product with higher caffeine content is the more likely source if it occurs.

What are the side effects of decaffeinated green tea extract?

The liver-related findings above apply regardless of caffeine content, since they're tied to EGCG and catechin dose, not caffeine. A decaffeinated product should avoid caffeine-specific effects like jitteriness or sleep disruption, but not the dose-related liver findings.

What are green tea extract side effects for females specifically?

None of the studies reviewed here reported outcomes broken out specifically by sex. There's no research basis here for a female-specific side-effect profile distinct from the findings above.

Related reading: how much green tea extract should you take a day?, when is the best time to take green tea extract?, and who should not take green tea extract?