Islam prevented humans of eating or drinking or using everything that is harmful for health and the more harmful the prevention of its cause would be more serious until it reached the level of prohibition.
Imam Khomeini (may Allah’s mercy be sent on him) said:” Eating what is harmful for man is forbidden.” It is clear that the criterion for being forbidden, is to be harmful, whether by eating or other ways, there is not difference. Cigarettes are one of those things that’s use is harmful for man’s health, but is this damage so much that using it would be forbidden or not? It depends on people and different conditions.
Ayatollah Kamene’i answered a question about this and said: “The rule with various levels of harms that followed by using tobacco, are different. And in general, if the using of tobacco was that much which caused a remarkable harm for the body it isn’t allowed. And if person knows that by starting smoking it will reach that level, isn’t allowed.”
Ayatollah Makarem Shirazi said about it: “The rule of eating or drinking something that has serious harm, is forbidden. If using cigarettes and other forms of tobacco are considered seriously harmful by those who have the knowledge than it is forbidden. Using drugs is completely forbidden; ether by injection or smoking or eating or any other way. Also the making of, traffic, and every form of assistance in its distribution is forbidden.”
Other maraje taqlid have the same view about it – that if it has serious harm it is forbidden. Ayatollah Safi said: “Eating something that is harmful for ones mind and health is forbidden, but if the harm is temporary in such that wise people don’t heed, isn’t forbidden.”
 Tawdih al-Masa’il, ruling 2630
 Ibid, ruling 1407
 Ibid, ruling 2630