I dunno whether or not it kills brain cells, but I highly doubt it kills any more than just living day to day would. However, I do experience a problem after smoking hard for a while. What happens is, my vocabulary starts to suffer. Not common words, but words I use less frequently. I'll know there *is* a word, I'll know that I know the word, and I'll know it's appropriate to use the word, but on the way to retrieve the word from its mental compartment my brain gets lost and I can't cough up the word I want.

If I quit smoking for a month or two, my vocabulary repairs itself, and eventually returns to normal. Because it *does* go back to normal, that's pretty good evidence that whatever is going on is temporary. It is somewhat annoying though, to lose access to parts of my vocabulary. It is the one unambiguously bad side effect of pot for me. It's not a major deal, but I wish it didn't do that.