gregaaron:
Please be corrected on the addictiveness of alcohol. There is a physical dependency that is among the most significant ones medically. Withdrawal from alcohol must be medically managed, as it is potentially fatal. The development of a physical dependency is statistically more likely when it is being used for its mind altering effects. But even if not, the addiction can happen. Because of how severe the risks can be, it is unwise to minimize this aspect of it. In my earlier comment, I noted that the casual drinking which is much less likely to lead to physical dependency is debatable, and I would side against it being allowed for bochurim.
You are absolutely correct in the addictiveness of a single cigarette. Actually, research indicates that it takes 4 cigarettes to establish dependency, and tobacco manufacturers actually handed out free packs of 5 cigs each many years ago to establish their customer base.