The presumption is that without clear evidence that something is prohibited, it is permitted. However, I am more inclined toward the school which finds a reasonably strong rationale for the prohibition of smoking. This school infers the Quran verse: "[when] they ask you about wine and gambling tell them that there are advantages as well as harms in them for the human and their harm is considerably greater than their advantages." The rule that can be inferred from this verse is that anything that has a greater harm than advantages is prohibited. All physicians testify that smoking is harmful to the person; therefore, if he or she is aware that smoking will lead to his or her death, it is prohibited for him or her to smoke.
We tend to advise everyone to consider all the drawback of anything they are about to get used to, before they actually get used to it. It is reasonable to avoid something there is no real benefit in it, even though one might have got used to it. A reasonable person would not endanger himself or herself, and would not continue a harmful habit.
یت اللّه فضل اللّه : اصل در اشیاء و اموری که نص صریحی در باره آن در دست نداریم، اباحه (مباح بودن) است، اما من به قولی نزدیکم که وجهی مناسب برای حرمت استعمال دخانیات می بیند و از این آیه الهام می گیرد که می گوید: