It's funny, I was thinking about this just the other day... which drug is the most harmful to the brain/causes the most number of irreversible changes to the brain.
I came to the conclusion that, both in my experience and based upon what I know, stimulants are probably the most likely to cause the most number of irreversible changes to the brain as certain stimulants cause the dopamine receptors to "curl" or "die" ...
I'm not entirely sure about benzos and what irreversible changes to the brain, if any, benzos might induce, but I do know that I suffered a withdrawal seizure after stupidly kicking benzos cold turkey -- not fun!
I'd imagine that opiates are actually the least likely, are one of the substances least likely, at least, to cause irreversible changes to the brain. It seems to me that, while opiates do undoubtedly change the way the body functions (constipation, and the horrid withdrawal symptoms, of course...) ... changes to the brain ? Not so much, I'd reckon... although I am sure that there are those here who will disagree with me on that one.