Ok everyone listen!
The combustion of organic matter (burning plants) produces carsonagenics.
Carsonagenics cause cancer.
Therefor smoking weed gives will eventualy give you cancer.
But the chance of getting cancer increase with the amount you smoke, the action of other chemicals in the weed (there have been sugestions that THC counters cancer).
In my oppinion if you smoke a joint a day for 50 years the chance of getting cancer is low enough to justify you doing it. But do your research as new developments happen every day.