As far as I know it has never caused cancer, however inhaling any kind of smoke is not good for the lungs. I remember reading an article claiming that although cannabis created more pre-cancerous lesions on the lungs, these were 'pre-cancerous' and never developed into actual lung cancer. These rips in the lung also occur with tobacco smoke, but these lesions become cancerous. This is because of the radiation given off from tobacco due to the company's use of the cost efficient high phosphate fertilizers which contain the radioactive elements lead 210 and polonium 210 (for English/Russian folks, this is the poison used on Litvinenko in a much higher concentration). Anyway, for more info just search tobacco radiation on google...quite a lot of articles on it