Is cannabis a stepping-stone for cocaine?

This paper uses a unique dataset on the inhabitants of Amsterdam, to study the dynamics of the consumption of cannabis and cocaine. People are most likely to start using that drugs at ages 18-20 and 20-25. An analysis of the starting rates shows some evidence of cannabis being a "stepping-stone" for cocaine. However, the fact that some individuals use both cannabis and cocaine has to do mostly with correlation through (unobserved) personal characteristics and not with cannabis causing the use of cocaine.