Green tea is the fourth most commonly used dietary supplement in the United States, according to the authors of an article published in the journal "Drug Safety" in June 2008. Although it contains a high concentration of potent antioxidant compounds called polyphenols, taking too much green tea extract can cause detrimental side effects, the most dangerous of which is liver damage.

Dosage

After reviewing decades of case reports, the U.S. Pharmacopeia Dietary Supplement Information Expert Committee published its findings in June 2008 in "Drug Safety." The article warned consumers that using large amounts of green tea extract, particularly on an empty stomach, is more likely to cause adverse effects than drinking traditional green tea. According to the University of Maryland Medical Center, the standard recommended adult dosage for green tea extract is 100 to 750 milligrams a day. Intakes above that may be toxic.

Liver Problems

Some European countries have placed restrictions on sales of green tea extract supplements because people have developed liver problems after taking them, according to researchers who conducted a study that was published in "Food and Chemical Toxicology" in February 2005. To test this claim, the researchers gave green tea extract to rats and found that high concentrations caused toxicity in the rats' liver cells. This led the researchers to conclude that excessive green tea extract may lead to liver damage.

Other Organs

Taking large amounts of green tea extract may also have negative effects on other organs, according to the authors of an article published in "Toxicologic Pathology" in 2010. When the researchers gave the extract to mice for 14 weeks, they found that concentrations above 1,000 milligrams per kilogram of body weight were lethal. Although the deaths were likely due to liver necrosis, the green tea extract also caused secondary adverse effects to the animals' noses, mesenteric lymph nodes and spleens.

Caffeine