Source: Bill Briggs, "Dude, it's your junk! Pot linked to testicular cancer," NBC News, 10 September 2012.
"Scientists at the University of Southern California say they've detected a link between recreational marijuana use and a greater chance among males in their early teens through their mid-30s of contracting a particularly dangerous form of testicular cancer -- non-seminoma tumors, according to a small study published today online in CANCER, a peer-reviewed journal of the American Cancer Society. "The group that is at risk for developing these tumors is overwhelmingly young men. They should be looking and paying attention to changes in their testicles anyway," said Victoria Cortessis, one of the study's authors and an assistant professor of preventive medicine at the Keck School of Medicine of USC in Los Angeles."
"... men who had used marijuana recreationally were twice as likely to develop mixed-germ-cell tumors, including the deadlier non-seminona tumors. ... "These tumors usually occur in younger men and carry a somewhat worse prognosis" than other types of testicular cancer, the study reported. Moreover, the USC findings confirmed two previous reports in CANCER of an apparent link between marijuana use and cancer of the testicles, the researchers noted."
This is one particular type of cancer, but it does provide evidence that marijuana is not necessarily as safe as its proponents wish to make it out to be. While this study was relatively small, it should be noted that this is the third study to draw the same conclusion.