It's not a myth according to these studies.
Cannabis smoke contains numerous
carcinogens.
[5][6][7] Surprisingly, an extensive study published in 2006 by Donald Tashkin of the
University of California, Los Angeles found that there is no significant link between smoking cannabis and lung cancer.
[8] The study, which involved a large population sample (1,200 people with lung, neck, or head cancer, and a matching group of 1,040 without cancer) found no correlation between marijuana smoking and increased
lung cancer risk, with the same being true for
head and neck cancers as well. The results indicated no correlation between long and short-term cannabis use and cancer, indicating a possible therapeutic effect. Extensive cellular studies and some studies in animal models suggest that THC or
cannabidiol has
antitumor properties, either by encouraging
programmed cell death of genetically damaged cells that can become cancerous, or by restricting the development of the blood supply that feeds tumors, or both.
[9] Unlike most other studies, this one had a very large sample size and was controlled for tobacco, alcohol, and several socio-demographic factors, which likely confounded the other studies.[
citation needed]
Prior, a 1997 study examining the records of 64,855 Kaiser patients (14,033 of whom identified themselves as current smokers), also found no positive correlation between cannabis use and cancer.
[11]
http://en.wikipedia.org/wiki/Cannabis-associated_respiratory_disease#cite_note-10
http://en.wikipedia.org/wiki/Cannabis-associated_respiratory_disease