A Johns Hopkins research team reported in 2000 that HPV infection is associated with some head and neck cancers, such as the upper throat and oropharynx. In their study, HPV16 was found in 92 of 240 individuals with head and neck squamous cell carcinoma.
The research team recently found that marijuana use was associated with increased prevalence of head and neck cancer in individuals with HPV, but that tobacco and alcohol was not. They reported that people who smoked pot for > 5 yrs were 11x more likely to develop cancers that tested HPV-positive. At the same time, they also said that the association was stronger with increasing numbers of oral sex partners, so that could be a significant confounding factor.
If it is pot that's increasing the risk of developing H&N cancer, the research team suspects that cannibinoids may inhibit the immune system's ability to clear infection; if pot were an immunosuppressant, you would see an increased prevalence of lots of infections in pot smokers.
Read more about it here: http://www.sciencedaily.com/releases/2008/03/080311165905.htm