I really can't function on a normal level without at least one morning coffee, and another by about 1 PM, and again at about 4 or 5 (speaking of...). I've cut back a lot, I used to have about eight to twelve cups a day last time I was out of work (nothing better to do but play Civ for 19 hours at a time, and drink coffee). I'm down to usually no more than four cups a day, now. I successfully gave it up for a time, and then found out that due to an unrelated blood disorder, going without the coffee was actually doing me more harm than drinking too much of it, so I got myself back to four cups a day.
I don't smoke pot, but I know quite a few who do. Only one of them is your traditional dazed stoner, and I strongly suspect he's just an idiot. The others are quire functional members of society. I have known three people to be diagnosed with lung cancer from smoking tobacco, though (strangely enough, not the pack a day smokers I live with...)