Here's a study, that I missed where they PAY you $600 to smoke 3 joints a day (you know it can't be very good pot if you can smoke 3 whole joints a day - it was grown by the Fed at their pot farm and had "moderate" THC content - probably the same as low grade Mexican pot)!

http://news.ucsf.edu/releases/ucsf-testing-medical-marijuana-for-cancer-pain/

They also did one in 2006 for the vapor form of it.

Needless to say, medical use of marijuana is controversial and some studies claim a link to H&N cancer although I haven't been able to find any numbers or statistics. It would make sense that anything that irritates the head and neck area could have a mutagegenic effect, alcohol containing mouthwash, Nyquil, hot drinks, even excessively cold drinks I have read (to name a few).
I have a agree with Eric that you can find a study that will tell you what you want to know. The double blind, scientific unbiased studies are what is really needed.