About 1/5 or 1/4 of smokers get lung cancer, and a large proportion of smokers will die sooner than non-smokers, due to various other smoking-induced causes, from leukemia to heart disease to pneumonia. The proportion is probably more than 50%, though you’d have to analyze a lot of research, and have a good understanding of statistics and research methodology, to demonstrate this.
I know that some poskim think that it matters whether there’s more than a 50% chance of death from smoking. But I don’t.
If you play Russian Roulette once, with just one bullet in a six-cartridge revolver, there is only a 1/6th chance of dying. But it is certainly forbidden, because it is akin to suicide. So is smoking. Smoking a single cigarette should also be asur because it can lead to a habit and then an addiction.