There are difficulties in researching the effects of cannabis.
Links
[1] http://www.cancerresearchuk.org/about-cancer/cancers-in-general/cancer-questions/does-smoking-cannabis-cause-cancer#why
[2] http://rethinkpot.org/tags/research
[3] http://rethinkpot.org/tags/cancer
[4] http://www.cancerresearchuk.org/about-cancer/cancers-in-general/cancer-questions/does-smoking-cannabis-cause-cancer#carc