Tea is the most popular beverage worldwide (after water), but coffee is close behind it in industrialized countries. While caffeine is a natural component of both tea and coffee, coffee contains more of it: anywhere from 60 to 120 milligrams in six ounces, depending on brewing methods and other factors. Caffeine is a mild psychoactive substance—it stimulates the central nervous system and improves alertness. It also boosts the analgesic effect of aspirin and other pain relievers, which is why it's added to some of these drugs. In large enough amounts, it can cause heart palpitations, stomach upset, and insomnia.

Coffee, derived from a bean, contains many other phyto-chemicals besides caffeine, and some of them may have beneficial effects in the body, as do those in tea, though this aspect of coffee is only beginning to be studied. Coffee itself has been extensively studied for years, generally to see if it causes disease. While many people believe decaf is more healthful than regular, there's no proof that it is.

Coffee has been blamed for causing many ailments, but in nearly every instance, it has been declared not guilty.