Green tea is safe for most adults when taken in moderate quantities, according to WebMD. Drinking more than five cups of tea per day can be dangerous, inducing side effects or even becoming fatal. Side effects include diarrhea, headache, heartburn, vomiting and convulsions.
The caffeine in green tea causes most of the possible side effects. Nonetheless, when consumed properly, green tea has many health benefits, including increased mental health, increased heart health, improved blood flow, lower cholesterol and enhanced physical performance. Green tea is high in antioxidants called catechins, which fight against cell damage. Green tea may also help prevent Alzheimer's disease, according to WebMD.