Just my humble opinion, so please don't take it too seriously...
I personally would say that smoking is a sin. The reason is this: no matter how much one smokes, it is harmful to the body. Even one cigarette is harmful. The purpose of smoking is only for pleasure. Doing something ONLY for pleasure that HARMS the temple of the Holy Spirit would, in my opinion, be a sin. Now, I saw an episode of House (love that show!) where he prescribed one cigarette a day to a man for some ailment (I forget what). If it were true that cigarettes actually help (as in the case of marijuana for cancer and glaucoma), then I would say that for that purpose, it is okay out of economia. Kind of like birth control-- it's okay when it is for the purposes of medical treatment of an ailment or hormonal imbalance.
As far as drinking alcohol, many people enjoy it for the taste, as a beverage. In moderation and for this purpose, I would say that alcohol is perfectly fine. But overindulging and abuse is, of course, a sin.
Both of these are addictive. In the case of addiction, I would say that it is definitely a sin, as we should never be addicted to something. It is an abuse of the body, and it makes an idol out of that substance, as we are constantly seeking it, rather than God.
This is just my opinion, though, and I am certainly not a priest. I would echo the sentiments of HandmaidenofGod and say that, as always, a spiritual father should be consulted in these matters.
Christ is Born! Glorify Him!