A large-scale study has concluded that regular, heavy pot smoking (a lifetime smoking more than 22,000 joints!) does not lead to lung cancer. Snip from WaPo:
The new findings 'were against our expectations,' said Donald Tashkin of the University of California at Los Angeles, a pulmonologist who has studied marijuana for 30 years. 'We hypothesized that there would be a positive association between marijuana use and lung cancer, and that the association would be more positive with heavier use,' he said. 'What we found instead was no association at all, and even a suggestion of some protective effect.' Link.