Actually, there have been studies that show weed (by volume) is more cancerous than cigarettes. The difference is people will smoke 10+ cigarettes a day by themselves (i.e. they don't share) for years on end whereas most potheads only smoke maybe 4 per day and they tend to share them (puff, puff, give).
Weed isn't physically addictive AT ALL. Physical addiction means that your body literally adjusts to and depends on a substance to function properly. This doesn't happen with weed. It CAN be psychologically addictive, but so can food, exercise, sex, and virtually anything else. If you have a psychological addiction, you'll get cranky and aggravated if you haven't got a fix, but physical addiction means that you'll get physical withdrawals (stomach aches, headaches, itchy skin, etc.). Physically addictive substances include cocaine, crack, heroine, meth, caffeine and alcohol, and, aside from caffeine, all of these can have such severe withdrawals that you can die from them. (In other words, you can die from "alcohol deprivation".)