Nowadays, since there are studies that show that smoking is dangerous to health, is one allowed to smoke or is it violating "Venishmartem Meod Lenafshosechem"?
There is a machlokes haposkim on this matter.
Igros Moshe wrote in 1964 that since there is a possibility of danger from smoking, one should refrain from doing so, however, it is not forbidden because "Shomer Psaim Hashem", Hashem guards the fools.
In 1981, R' Moshe wrote that the danger of smoking is no more than the danger of eating unhealthy foods, about which the Rambam does not write that they are "forbidden", just unrecommended. Therefore, he writes, because people get withdrawal syndromes when they do not have what to smoke, and the danger is not so great (R' Moshe writes that the amount of people who get sick is a small minority). Therefore, we say that "Hashem guards the fools".
However, he says that one should not start smoking, since there is some danger without any benefits, and there is no desire to smoke until one starts.
However, R' Moshe Shternbuch said that it is forbidden because of
- danger from Cancer and heart problems.
- the Chazon Ish said that smoking hurts memory and comprehension, so smoking would be prohibited because of "Lo Tishkach", that one isn't allowed to do something to cause oneself to forget words of Torah.
Binyomin Zilber takes a middle approach, and says that while it may be very unhealthy, yet it would not be intrinsically prohibited. He brings a proof from the Mishna Brurah which permits smoking on Yom Tov, yet in his Likkutei Amarim he says that smoking weakens a person, and causes a waste of time and money. In other words, there are things that are wrong to do, yet not officially "prohibited".
The Previous Lubavitcher Rebbe writes (in 1943) to the Rosh Yeshiva of Tomchei Tmimim to absolutely forbid smoking to students under 20 wherever they may be for all 24 hours a day. He should also give over a request to students who are above 20 that it is in their benefit (both physical and spiritual) to quit smoking, by slowly lessening their smoking until they will quit completely.
There are two opinions as to what was the Lubavitcher Rebbe's opinion. Someone asked the Rebbe why do Rabbonim not forbid smoking. The Lubavitcher Rebbe answered with two reasons:
- This matter is for doctors to decide, not Rabbonim.
- It is possible that a harmless cigarette will be invented. However, if there will be a ban, that cigarette would also be included and would be needlessly forbidden.
R' Oberlander writes that one reason to permit is because of "Hashem guards the fools". One of the sources that we can rely on the verse "Hashem guards the fools" is a Gemara in Yevamos(12b) that has an argument between R' Meir who says that a 11-12 year old should prevent pregnancy (because it is dangerous to give birth at that age) and the Chachamim who permit it because "Hashem guards the fools".
However, he brings down the Tzemach Tzedek who asks if we can say "Hashem guards the fools" in cases where a pregnancy is dangerous for a woman. The Tzmeach Tzedek says that one could differentiate between the Gemara's case and a dangerous pregnancy, because in the case of the Gemara, most 11-12 year olds don't get pregnant, so we could rely on "Hashem guards the fools" and a "majority" to assume that the dangerous situation won't come up and there will be no pregnancy. However, if the danger is more definite, one can't say "Hashem guards the fools".
R' Oberlander then goes on to say how smoking is not very dangerous, so we could rely on "majority" (majority of people who smoke don't get sick) and "Hashem guards the fools" to assume the "minority" dangerous situation won't happen and permit smoking.
However, R' Zayents explains the Tzemach Tzedek differently. He explains the idea of "Hashem guards the fools" is that when it applies Hashem prevents the "minority" dangerous case from happening. In other words, by a 11-12 year old, "Hashem guards the fools" means that such a case will not occur (she won't get pregnant). However, if a dangerous situation would occur, we would have to worry about it in the first place. Since in smoking there is at least this "minority" of smokers who get sick, we would have to take them into account and forbid smoking. Moreover, R' Zayentz points out that the letter where the Rebbe gave reasons why Rabbis don't forbid smoking does not mention "Hashem guards the fools". The Rebbe's letter says that smoking is a medical questions for doctors to address, and as long as it's unhealthy it's automatically forbidden, and when it will become healthy, it will be automatically permitted.
Interestingly enough, Igros Moshe permits the Dor Yeshorim test even though there is only a small chance of having children with birth-defects, and says that not taking the test is like closing one's eyes to danger.
To add to the answer(s) about R' Moshe Feinstein's Pesak, R' Dr. Moshe D. Tendler, son-in-law of R' Moshe Feinstein, has been an outspoken advocate for banning smoking 'Al Pi Halachah, and he has publicly stated (in fact, I've been present in a large audience where he has stated) that he firmly believes R' Moshe Feinstein's Pesak would have been very different, outlawing it 100% in all cases, had the medical community known then what it knows now about just how deadly it actually is.
One point he emphasized when I heard R' Dr. Tendler speak on the topic is that, at the time that R' Moshe Feinstein wrote his Teshuvah, it was believed by the medical community that those who get sick are a small minority and that the risk is small. R' Moshe consulted doctors and medical experts in rendering his decision. The consensus among them at the time of his decision was that it wasn't good for you, but it wasn't yet determined to be as severely or as commonly dangerous as we now know it to be. In other words, the wording of R' Moshe's ShU"T reflected the understanding of the medical community's overall consensus as conveyed to him by those experts he consulted at the time. According to R' Dr. Tendler, R' Moshe based the Pesak entirely on that assessment, which, according to him, means that the Pesak would have been a 100% ban with today's awareness in the medical community.
To echo above answer:
Rabbi Moshe Feinstein strongly discouraged it and prohibited new use, but couldn't quite prohibit existing smokers from continuing their awful habit (and as such, allowed one to give a smoker a light if asked), despite protests from his student Dr. Fred Rosner.
Some 15 years later, in light of further medical proof of just how awful smoking is (and the advances in detection and treatment for many types of cancer, but not lung), the RCA's position became to flat-out prohibit it.
No one has brought up this angle, but second hand smoke is one of the most common asthma triggers which means that if you are smoking, even outside, or even in your own home, and someone with asthma happens to come by and gets triggered - that can be fatal.
Just like we forbid leaving fingernails where they can cause a miscarriage, the same thing should apply here, unless it is as clear that an asthmatic would not be around as it was clear that a woman would not be in a Beis Medrash, it would seem that smoking would be problematic regardless of the personal health danger to the smoker.
Not to stick my head between these great mountains but i have a Rambam that noone mentioned. Its in chapter 5 of shmoneh prakim, which the Rambam sends you to on the mishna' viyihiyu kol maseichem lisheim shomayim'. I won't attempt to summarize, but I would guess from his words that if someone smokes in order to calm his nerves so he can be a nicer, friendlier person and so he can have a clear head to learn, than he is correct in his actions. And if someone quits so he can live longer and do more mitzvos, he is correct in his actions. But someone who quits for the selfish desire of living longer is not respectable.