The possible side effect of green tea contains high concentration of catechin polyphenols and other free radicals which are responsible for triggering the adverse effects on the body. Green tea extract is considered very beneficial in many health conditions and it is also used in natural remedies as well. Some common side effects of green tea include allergies, insomnia, heart attack, headache, and weight loss. These side effects of green tea are more pronounced in people who consume it in large quantities on a daily basis. It is advised to consult a doctor before using green tea as it has certain medicinal properties but also has hazardous properties.