The Jewish people are commanded in the Torah to take care of their physical well being. The Torah (Debarim 4:9,15) states explicitly that one must keep a close watch over his health. Maimonides (Hilchot Rotseach 11:5 - 6) writes that one who engages in unhealthy activities and declares that he has the right to do as he wishes to his own body deserves a punishment. The Talmud (Hullin 9a) states explicitly that one must treat dangerous activities with greater stringency than one would treat Halakhically prohibited activities.
In modern days, most (if not all) Orthodox rabbis consider smoking cigarettes as a Halakhically forbidden activity.
Why?
Cigarette smoking causes a variety of life-threatening diseases, including lung cancer, emphysema, and heart disease. An estimated 400,000 deaths each year in the U.S. are caused directly by cigarette smoking. Smoking is responsible for changes in all parts of the body, including the digestive system. This fact can have serious consequences because it is the digestive system that converts foods into the nutrients the body needs to live.
Rabbi Ovadia Yosef points out that just as we rely on the wisdom of doctors to permit doing otherwise forbidden activities on Shabbat or eating on Yom Kippur, so too we are required to listen to them and distance ourselves from those activities they deem dangerous, like smoking.
For a full Halakhic discussion and a modern Rabbinic ruling on smoking, click here.