Green tea extract is a concentrated form of green tea that's made from the leaves of the Camellia sinensis plant, which is native to Asia. It contains antioxidants, including flavonoids, vitamins, and catechins, as well as caffeine. The most effective antioxidant compounds are the flavanol monomers known as catechins, especially epigallocatechin-3-gallate (EGCG) and epicatechin-3-gallate.

Green tea extract may have many health benefits, including: 

• Antioxidant activity: Antioxidants can help protect cells from damage caused by certain chemicals that may increase the risk of cancer and other diseases. 

• Cancer prevention: Green tea extract may help maintain cell health and prevent some types of cancer, including those of the prostate, stomach, and esophagus. 

• Exercise performance: Green tea extract can reduce cellular damage and delay muscle fatigue. 

Green tea extract is available in liquid, powder, and capsule form. It's soluble in water, alcohol, and glycerol but is insoluble in oil. 

Green tea extract can cause side effects due to caffeine, including anxiety, tremors, irritability, and sleeping problems. It may also cause liver problems, with symptoms such as yellowing of the skin or the whites of the eyes, nausea, and stomach pain. However, side effects are less common with green tea than with other drinks that have caffeine because the leaves are steeped for a shorter time.