Caffeine is one of the most common drugs in the Western world.
Problems can arise via the intake of caffeine and, as with any drug, very high doses can be lethal and certain safe limits should be adhered to. Also, people can become dependent on caffeine if they drink coffee etc. Quitting caffeine can be as difficult as any drug. On a more scientific note, caffeine is not a particularly complicated molecule. The structure of caffeine involves 2 carbon-nitrogen rings connected together with oxygen atoms attached to each one.