I don't know if there have been any studies done on the long term effects of weed, but Alcohol can really fuck you up. When I decided it was time to grow up and cut back and focus on things that were important to me, drinking was the natural choice to give up. When i get drunk, I get drunk Its the Bishop's way. And that means that I lose two days of being efficiantly  productive. One for the drinking, and one for the hangover... and when party nights at your school are Tuesday, Thursday, Friday, Saturday and in my department Sunday... well, you can see why I cut back on drinking.
I've always had one rule of thumb when it comes to altering my consciousness. Its fine, as long as I don't let it get in the way of my studies. I usually don't usualy smoke until im done everything else, later at night, and I fell fine the next day, so I can still get stuff done. I don't lose out on productivity, but I still get to relax and have a good time with friends.