Ritalin. Computer graphic of a molecule of the stimulant drug methylphenidate (C14. H19. N. O2), marketed as Ritalin. The atoms (tubes) of the molecule are colour-coded: carbon (yellow), oxygen (red), hydrogen (white) and nitrogen (blue). It is a stimulant of the central nervous system, and is used to treat attention deficit disorder. This is a psychiatric disorder where young children become hyperactive, disruptive, defiant and aggressive.