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).
There have been no reports of lung cancer related solely to marijuana, and in a large study presented to the American Thoracic Society in 2006, even heavy users of smoked marijuana were found not to have any increased risk of lung cancer.
Unlike heavy tobacco smokers, heavy marijuana smokers exhibit no obstruction of the lung's small airway. That indicates that people will not develop emphysema from smoking marijuana.
http://www.drugpolicy.org/marijuana/factsmyths/#tobacco They cite their sources right under that fact.
The only "studies" that are done on marijuana are the ones where they lie and fake their way through it. Marijuana being illegal is a sham. Alcohol is far more dangerous, addictive, and reckless than marijuana yet it's legal. Cigarettes kill too many people to be legal. If you can find me one case where a person died because of marijuana related lung cancer, I will give you my first born child.