Islam prevented humans of eating or drinking or using every thing that is harmful for health ...
Do all the clerics say that smoking is forbidden?
Islam prevented humans of eating or drinking or using every thing 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.”
Source: http://www.islamquest.net/en/archive/question/fa1079