Science reports that Marijuana appears to carry no cancer risk. CNN coverage here.

Apparently if you compare cigarette smokers to pot smokers, the potheads don't get cancer at near the rate of the tobacco users. A major caveat though, it isn't normalized in any way to pack years, in other words, no one smokes as much marijuana as even moderate smokers can puff away on cigarettes. After all, a 4-joint a day habit would be pretty heavy for pot, but a nicotine addict may smoke between half a pack to 3 packs a day. Even if pot has 4 times more tar and pollutants, even the heaviest tokers might only be getting an exposure equivalent a minimal smoker.

It is not clear why marijuana is illegal and alcohol and cigarettes are ok. There is no physiologic withdrawal from marijuana, certainly nothing even close to cocaine withdrawal or alcohol withdrawal, which can be so dangerous as to lead to fatalities, or heroine or nicotine addiction, which both have painful withdrawal (but don't carry a risk of death).