Black tea has a high caffeine content and a good amount of theophylline substance. This combination can make you feel increased alertness. Additionally, it contains polyphenols that are antioxidants and can help protect your cells from DNA damage.
Regular consumption of black tea is believed to reduce the risk of:
Diabetes
High cholesterol
Kidney stones
Parkinson's disease
However, excessive consumption of black tea (more than 4-5 cups a day), as well as the combination with other products containing caffeine, can have side effects on your health, such as increased stress, rapid breathing, insomnia, frequent urination, arrhythmia, nervousness, tremor.
Always follow your doctor's advice.