The effects of smoking are small cumulative effects, meaning that it’s not “this one cigarette” that results in addiction or that causes lung disease.
Smoking on occassion doesn’t guarantee an addiction but on the other hand it’s difficult to pinpoint, and very ideosyncratic, the exact timing of onset of addiction or lung disease. There are poskim who poskin assur; other poskim don’t (the “minhug” of Chosson cigarettes is still going strong). My point of contention is the OTD labeling.
Unfortunately, many of us have done aveiros, and sometimes even habitually: lashon hara, shmiras einayim, being lax in betching, brachos or hilchos boreir, not properly declaring taxes, and on and on – that doesn’t make us OTD – it means we have a weakness that needs tikun. Unfortunately there are sufficient real OTDers out there, that we don’t want to broaden the parameters of OTD and create self-fulfilling prophecies.