Just to balance this out with what we DO know...   The country's leading researcher of marijuana and lung disease, UCLA's Dr. Donald Tashkin, conducted investigations over 30 years, initially believing there must be a causal relationship. But he finally concluded that smoking marijuana does NOT cause cancer or ANY other serious disease.

Tashkin said:  "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. What we found instead was no association at all, and even some suggestion of a protective effect."