When taken orally, EGCG has poor absorption even at daily intake equivalent to 8–16 cups of green tea, an amount causing adverse effects such as nausea or heartburn. After consumption, EGCG blood levels peak within 1.7 hours. The absorbed plasma half-life is ~5 hours, but with majority of unchanged EGCG excreted into urine over 0 to 8 hours. Methylated metabolites appear to have longer half-lives and occur at 8-25 times the plasma levels of unmetabolized EGCG.
A 2018 review showed that excessive intake of EGCG may cause liver toxicity. In 2018, the European Food Safety Authority stated that daily intake of 800 mg or more could increase risk of liver damage. The degree of toxicity varies by person, suggesting that it is potentiated by genetic predisposition and the diet eaten during the period of ingestion, or other factors.
Over 2008 to 2017, the US Food and Drug Administration issued several warning letters to manufacturers of dietary supplements containing EGCG for violations of the Federal Food, Drug, and Cosmetic Act. Most of these letters informed the companies that their promotional materials promoted EGCG-based dietary supplements the treatment or prevention of diseases or conditions that cause them to be classified as drugs under the United States code, while another focused on inadequate quality assurance procedures and labeling violations. The warnings were issued because the products had not been established as safe and effective for their marketed uses and were promoted as "new drugs", without approval as required under the Act.
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