The largest and most comprehensive study on the topic shows that smoking marijuana does not cause cancer, although it may cause other lung diseases (bronchitis, wheezing, and general difficulty in breathing).
The same study confirms the strong link between smoking cigarettes and cancer (as expected).
Several studies showed that marijuana can cause “pre-cancerous conditions” but there appears to be no actual link between marijuana and cancer, possibly because of some counterbalancing effect. It appears that this confusion is being used to spread the disinformation that marijuana does cause cancer.
In fact, some marijuana components may help in treating cancer - Fox article, actual study .
