We do know that smoking is unhealthy. And that, like tobacco, cannabis contains cancer causing substances. Therefore it would seem likely to increase cancer risk. But we need more research to know this for sure.
Links
[1] http://www.cancerresearchuk.org/about-cancer/cancers-in-general/cancer-questions/does-smoking-cannabis-cause-cancer#evidence
[2] http://rethinkpot.org/tags/cancer