Sept. 10, 2012 -- Smoking marijuana may affect a man's risk for testicular cancer.
A new study found that men who had smoked marijuana were twice as likely as men who had not to get an aggressive form of the disease.
Testicular cancer is the most common cancer diagnosed in men under age 45. It's also on the rise, says Scott Eggener, MD, a cancer surgeon at the University of Chicago who has studied the trend.
"No one really knows why," he says. "Everyone suspects an environmental exposure, but it's difficult if not impossible to prove."
A study released earlier this year showed marijuana use is also up, with 1 in 10 teens now smoking pot at least 20 times a month.
The new study, published in the journal Cancer, is the third in recent years to link marijuana use to the development of testicular cancer.
It compared 163 men with testicular cancer to 292 healthy men who were about the same age and race. All the men in the study were between age 18 and 36 when they were diagnosed.
Men who said they had ever smoked marijuana had more than twice the risk of aggressive testicular tumors, compared to men who did not smoke marijuana.
That was true even after researchers accounted for other things known to affect a man's risk, like having an undescended testicle.
Oddly enough, men who reported using cocaine had about half the risk of nonusers.
That doesn't mean cocaine benefits the testes, though.
In animal studies, cocaine has "really devastating effects on the testicles," says researcher Victoria Cortessis, MSPH, PhD, assistant professor of preventive medicine at the Keck School of Medicine of USC in Los Angeles. "They get smaller and smaller."
"I don't think cocaine is protecting the cells from cancer. I think it's more likely that it's killing the cells and therefore they aren't getting cancer," Cortessis says.
A doubled risk of cancer may sound pretty scary, but researchers caution that men who have smoked pot shouldn't panic.
That's because the odds that a man will get testicular cancer are pretty slim to start with. About 1 in 400 white men are diagnosed by the time they are 35, according to the National Cancer Institute. So even if you double that risk to 1 in 200, any one man's chances are still pretty low.
The study also doesn't prove that marijuana causes cancer.
In fact, the relationship the researchers found wasn't easy to explain. Men with lighter habits or who had given up pot smoking had a higher risk of testicular cancer than those who were current smokers or who reported heavier use.
Researchers don't think that means smoking more pot is actually safer.
Other studies, which were larger, found that cancer risk increased with the size of a man's pot smoking habit. The new study may simply be too small to show the same relationship.
Doctors aren't sure why marijuana may increase the risk for certain kinds of testicular cancer. The active ingredient in the drug, THC, is known to disrupt hormone signals in the body. That may put cells in the testes on a path to cancer. More research is needed before researchers can say for sure.
The bottom line, says Stephen M. Schwartz, PhD, MPH, an epidemiologist at Fred Hutchinson Cancer Research Center in Seattle, is that marijuana may not be as harmless a drug as some people think.
"The overall story is that there's an increased risk in marijuana users," says Schwartz, who was not involved in the research. "It's particular to the kinds of testicular tumors that are the most aggressive and therefore the most likely to put a man's life at risk." What's more, he says, the finding "is pretty consistent amongst these three studies, which is something we should be paying attention to."