I don't smoke, I haven't even tried it. Never have, never will. If I'm in the vicinity of someone who is smoking I start feeling queasy and sick. I think the medical problems associated with smoking speak for themselves.
I'm 19 and legally allowed to drink in Australia. Yes, I do drink on occassion. I've woken up very sick a couple of times and totally regretted drinking the amount I had the night before - I now avoid drinking in excess after those escapades. What does "on occassion" mean for me? Once a week to once a fortnight (on holidays) to a couple of times a month if that when I'm at Uni (this is usually because of parties) and even then it'll only be a couple of drinks and not to the extent of getting "trashed" (I hate losing control

).
I consider myself to be a "good" drinker if there is such a thing, I try to look after myself - I drink plenty of water, I don't drink alcohol with an empty stomache, I make sure I'm always in a safe environment etc.
I think alcohol is alright if done safely. Yes, there are associated medical problems (it can't be denied) but for the most part if you're sensible and don't have any pre-existing problems then you're chances are greatly reduced.
I don't think smoking or alcohol could ever be defined as "totally harmless". Those that would define smoking and alcohol as "totally harmless" would be the ones that I'd label as most at risk of encountering the problems as they're not sensible - saying they're totally harmless is just proof of that.