Health,
I am not promoting smoking or overeating. I am not even saying they are “okay.” Not everything that is a poor lifechoice is an issur. Rav Moshe says that people should eat healthy and not smoke, but he holds that eating unhealthy foods and smoking are not a violation of the mitzvah of hishomer l’cha u’shmor nafshecha.
I have seen some rabbonim argue that Rav Moshe would agree that smoking is assur today because the chances of it causing a smoker’s death is over 50% (at the time Rav Moshe wrote the percentage was believed to be much lower, about 14%). And that may be correct for being a regular smoker. But the occasional cigarette (or cigar, or pipe) is mutar (even if inadvisable).