Marijuana may double the risk of testicular cancer among young men, particularly tumors that are more severe

"This is a very consistent finding now that marijuana seems to be associated with the worst kind of testis cancer that occurs in young men ... (it) may well be causal," says Victoria Cortessis, an assistant professor of preventive medicine at the University of Southern California's Keck School of Medicine in Los Angeles. The rates of testicular cancer in men are increasing– as much as doubling every 20 to 30 years, Cortessis said. At the same time, marijuana use has increased, often in young males. "It may be that marijuana use is actually interfering with hormonal signaling in a way that disturbs function of the testis," she said. "That's a possibility - that's something that we can now formulate specific hypotheses about and try to understand."