To do some myth busting here, most people think of cannabis as pretty harmless. Research has shown that that is not the case at all. As reported by the BBC:
Smoking cannabis is infact 20 times more likely to give you lung cancer, than smoking cigarettes. Smoking a single joint, is the equivalent of smoking a 20 pack of cigarettes.
Reason for this being, that the cannabis smoker inhales deeply and keeps the smoke in his lung to gain as much from the high as he can. This way of smoking is very dangerous. He absorbs 4 times as much tar from the tobacco, and 5 times more carbon mono-oxide.
Problem here is, that the genetic and molecular security systems, cannot handle such an overload easily, and deal with it as if these poisons had been administered in smaller dosis over time. And so the risk of causing irreversible genetic damage multiplies further, to a factor 20.
Medicinal marihuana, given in pill form, does not convey the same health hazards. On the other hand, greedy smoking with instruments like bongs, waterpipes and you name it, may be even more dangerous. More high; more tobacco poisons to go along with it.