Regularly drinking green tea can help you weight and reduce your risk of several diseases, including diabetes, heart disease, and cancer. But, it has its own side effect due to the presence of caffeine. These side effects can range from mild to serious and include headache, nervousness, sleep problems, vomiting, diarrhea, irritability, irregular heartbeat, tremor, heartburn, dizziness, ringing in the ears, convulsions, and confusion.