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.
There is a debate between R' Oberlander and R' Zayants
as to the meaning of the Rebbe's answer.
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.