Caffeine is a stimulant drug. It is a xanthine alkaloid compound
that acts as a psychoactive stimulant and a mild diuretic
(at doses higher than 300 mg- see Relative content: comparison
of different sources) in humans. The xanthine core of the caffeine molecule contains two fused rings, a pyrimidinedione and imidazole. The pyrimidinedione in turn contains two amide functional groups that exist predominately in a zwitterionic resonance the location from which the nitrogen atoms are double bonded to their adjacent amide carbons atoms. Hence all six of the atoms within the pyrimidinedione ring system are sp2 hybridized and planar. Therefore, the fused 5,6 ring core of caffeine contains a total of ten pi electrons and hence according to Hückel's rule is aromatic.