I'm not sure about that. I know a few people (n<10) that have used soft drugs over a longer time and I remember them as crazy sharp in their younger years and they are very wooly headed now. Concentration problems, simply seem less smart to me. Note that they are heavy users, but still I definitely see a difference there that does not correlate with other people that have gone through a similar stretch of time and that have simply gotten older, but that don't feel less smart, and those don't seem to have concentration issues either.

I'm sure there is a scientific story out there to either confirm or deny this, it's just a personal observation.