Protection against dehydration A perhaps unexpected benefit of tea drinking is the resulting increase in water consumption, which protects against dehydration. Two cups of tea are approximately equivalent to one cup of plain water in their hydrating effect. Many other unwanted effects are associated not so much with use as with withdrawal. Prominent among these effects is the withdrawal headache, caused by dilation of blood vessels in the head that had been constricted by the consumption of caffeine. Other side effects of withdrawal, usually found in frequent and heavy users of caffeine, are lethargy, irritability, and constipation.
All real tea contains caffeine unless the tea has been artificially decaffeinated.
The amount of caffeine in a cup of tea varies tremendously, depending on the variety of tea and the brewing time. Although green tea often contains less caffeine than black, in some cases it may have just as much or more.