This is actually a very interesting question that I’ve been looking into recently. It’s interesting because it’s one of those rare “grey areas” of morality in the Catholic Church. I’ve looked at a lot of threads on the boards, asked several priests, and this is what I conclude (although don’t take this as definitive).
Smoking is not intrinsically evil. To smoke a single cigarette will NOT give you cancer nor pose much individual risk to a person. It is not a sin to do so, and so it is no more a sin than having a glass of wine or a beer. However, because of the serious cumulative effects of smoking, to smoke to excess would be a venial sin, just as drinking to the point of intoxication is a sin. Both involve gluttony and harm to the body. BUT… here’s where it gets fuzzy… Smoking is also highly addictive, so once it got to the point of excess, you would also likely be an addict, and that removes much of the culpibility. So, it might not be a sin in that circumstance. Further, most people start smoking when they’re young, and in order to sin, one must understand that it is a sin. Many youth are not fully aware of the serious health consequences of smoking, and so the seriousness of the sin might be reduced, if not removed entirely. So, given the age that most people start, the fact that smoking in moderation is not a sin at all, and that addiction removes much of the accountability, it’s likely that a large number of people aren’t sinning when they smoke.
There are, of course, circumstances in which smoking most likely remains a sin. Smoking while pregnant, for example, would be a sin. Spending excessive money on smoking to the detriment of other things is a sin. Second-hand smoke may or may not be a sin, I haven’t really figured that one out yet.
