Anyone who thinks that taking significant quantities of mind altering drugs for a long period of time will not effect their mental state, is just a silly naive fool IMHO

The extent though to which it does so is likely to be very hard to assess, some would argue that often the sort of people who feel the need to turn to drugs for a good night out are the same sort of people who would have got depression anyway, so things like that can effect the results of tests.

Certainly would be good to get some real data, but it NEVER seems to happen for ANY illegal drug and even on legal recreational drugs the information is pretty poor really.