has significantly higher amounts of some known carcinogens than tobacco.

"Marijuana smoke contains about 50% more benzopyrene and nearly 75% more benzanthracene, both known carcinogens, than a comparable quantity of unfiltered tobacco smoke (Tashkin, 2013). Moreover, the deeper inhalations and longer breath-holding of marijuana smokers result in greater exposure of the lung to the tar and carcinogens in the smoke. Lung biopsies from habitual marijuana-only users have revealed widespread alterations to the tissue, some of which are recognized as precursors to the subsequent development of cancer (Tashkin, 2013)."



If I had to guess at why they haven't proven a connection yet, it is probably because of "relatively small number of long-term heavy users – particularly older users" of marijuana. As it becomes legal, that number will increase and I imagine we will see similar effects.

In any event, the authors conclude:

"[E]ven though population-based studies have generally failed to show increased cancer risk, no study has definitively ruled out the possibility that some individuals, especially heavier marijuana users, may incur an elevated risk of cancer. This risk appears to be smaller than for tobacco, yet is important to consider when weighing the benefits and risks of smoking marijuana. (Tashkin DP, 2013)."