One common source of caffeine is the coffee plant, the beans from which are used to produce coffee. Caffeine content varies substantially between Arabica and Robusta species and to a lesser degree between varieties of each species.
One dose of caffeine is generally considered to be 100 mg. In theory, a single serving (6 fl oz / 150 ml) of drip coffee or one-half caffeine tablet would deliver this dose. In the real world, coffee varies considerably in caffeine content per serving, ranging from about 75 mg to 250 mg. Generally, dark roast coffee has less caffeine than lighter roasts since the roasting process reduces caffeine content of the bean.
Tea is another common source of caffeine in many cultures. Tea contains somewhat less caffeine per serving than coffee, (usually about half as much, depending on the strength of the brew),though certain types of tea, such as Lapsang sou chong smoked teas, and oolong contain more caffeine.
Caffeine is also common in soft drinks such as cola. Such drinks typically contain about 25 mg to 50 mg of caffeine per serving. Some "energy drinks" such as Red Bull contain 80 mg, while others offer considerably more caffeine per serving, from 100 mg to 400 mg.
Mateine and guaranine are other names for caffeine. The names come from yerba maté and guarana respectively, caffeine-containing plants used for tea and other things. Many yerba maté enthusiasts insist that mateine is a stereoisomer of caffeine and thus a different substance altogether. However, this is impossible;caffeine is an achiral molecule with no chiral centers, and therefore has no stereoisomers. Similar claims are sometimes made of guaranine.
Caffeine is sometimes called theine when it is found in tea, as the caffeine in tea was once thought to be a separate compound to the caffeine found in coffee. But tea does contain another xanthine, theophylline whose chemical structure is C7 H8 N4 O2 compared to caffeine's C8 H10 N4 O2. This is similar to the naming problem with mateine and guaranine.