At the end of the day, pretty much everything damages your brain cells. Breathing oxygen, poor diet, just time/ageing itself. And brain cells don't regenerate after your early 20's.
So, a narcotic, especially one that is so psychoactive, is pretty much going to do the same.
But compared to say, alcohol, which annihilates brain cells in droves, or frequent bashes to the head by boxing, it's fairly mild.
But then that can depend on usage as well.