Cigarette smoking is a form of self-induced toxic poisoning. So why did the greatest American authority on Jewish law (poseq), R. Moshe Feinstein, not declare tobacco smoking as prohibited? As Hirhurim points out, R. Feinstein did forbid marijuana. I’ve reviewed some of the comments on Hirhurim who criticize R. Feinstein on tobacco. Now let’s look at blog commenters and others who defend R. Feinstein’s approach. As noted previously, Reb Moshe said (1963, 1981) that smoking is not prohibited as self-endangerment because, given that so many indulge (dashu bo rabim), "The Lord preserves the simple" (shomer peta'im ha-Shem). Defenders mostly re-apply R. Feinstein’s approach so as to prohibit smoking:
A. The informed judge. Some argue that, if only R. Feinstein had known the extent of the health risks, he would have prohibited smoking. This is plausible and has long been advocated by his son-in-law, R. Moshe Tendler. (See D. below and ). However, I would point out that R. Feinstein had good access to health evidence in 1981 and still upheld that smoking is permitted under Jewish law. Indeed, R. Feinstein deliberately dismisses the health evidence as (potentially) erroneous.
B. The informed smoker. As long as smokers become aware of the serious health risks of smoking, they no longer qualify for the exemption that “The Lord preserves the simple”. Rabbi J. David Bleich recently argued that R. Feinstein’s 1963 ruling “accurately reflects the societal reality of that time… However, it is more than likely that, at present that condition no longer obtains.”  Thus, with mandatory package warnings (U.S.) and anti-smoking campaigns, most people do not take smoking risks for granted. (i.e., the dashu beh rabim exemption is gone)
C. Cumulative harm. A few commenters pointed out that it’s difficult to argue that any given cigarette causes harm. True, one cigarette may lead to many more. But a single cigarette cannot be forbidden as self-endangerment (e.g. Gil). Conversely, Mar Gavriel said: “I'm not sure that that's correct. For one thing, nicotine is inherently a poison. And for another, the act of inhaling smoke poses imminent danger to one's health. It won't be permanent, irreversible damage, but it will cause damage.” I would note that neither side clarifies the level of habitual smoking that might be prohibited under Jewish law.
D. Long-term harm. An anonymous blogger noted that Jewish law cannot easily forbid an uncertain harm that may be caused over the long haul. I would add that R. Bleich distinguishes between immediate and long-term risks (based on Binyon Zion 137). Since smoking involves long-term risks, R. Bleich argues that smoking is permitted as long as less than 50% of smokers are not irreversibly harmed. In 1977 and 1983, R. Bleich used this threshold to justify R. Feinstein’s conclusion. However, recently R. Bleich noted “that the cumulative risks … foreshorten the lives of the majority of smokers.” 
E. A defect in rabbinic authority. Mar Gavriel raises the question of whether Rabbis nowadays have authority under Jewish law to forbid smoking through a precautionary measure (gezerah). Indeed, R. Bleich (1977, 2003) asserts that modern Rabbis cannot proclaim a new health-based issur. Nevertheless, now even R. Bleich concedes that smoking is forbidden by the approach taken by R. Feinstein.
F. Halakhah responds to the community. Aramis argues: “What R. Moshe wanted to do was … not issue a pesak to the community which would be observed mainly in the breach.” Aramis is correct that halakhic principles that discourage rulings that the observant Jewish community cannot abide. Thus, Reb Moshe may permit smoking as long as people cannot abide by a prohibition.  This hypothesis cannot be proven but R. Feinstein’s 1981 obiter dicta lend credence to this notion.
Well, I've tried to do justice to the blog debate on Hirhurim, which I'd missed. Hopefully, I can wrap up my analysis later (by editing or with another post). Meanwhile,
 Tradition 2003 (37:3) p.96-7,
 Alternatively, R. Feinstein’s position may be explained by halakhic principles that authorize Rabbis not to publicize certain rules. If so, he may privately consider smoking forbidden but feel that it is inappropriate to say so. This principle can be invoked to protect the overall integrity of halakhah.