Coke isn't actually a prohibited substance out of competition, but because the test took place during the Tour it counts as in competition (when a bit of blow could certainly help). Presumably it's trace amounts rather than him doing bumps during the race.
http://www.lequipe.fr/Cyclisme-sur-route/Actualites/Luca-paolini-katusha-positif-a-la-cocaine/573363
Coke isn't actually a prohibited substance out of competition, but because the test took place during the Tour it counts as in competition (when a bit of blow could certainly help). Presumably it's trace amounts rather than him doing bumps during the race.