They found people who said they had used the drug were twice as likely to have been diagnosed with the disease as were never-users. The link appeared to be specific to a type of tumor known as nonseminoma.

"This is the third study consistently demonstrating a greater than doubling of risk of this particularly undesirable subtype of testicular cancer among young men with marijuana use," said Victoria Cortessis of the University of Southern California, Los Angeles, who led the work.

While the research isn't ironclad proof that marijuana harms men's nether regions, Cortessis said she believes young men should know about the link.

"I myself feel like we need to take this seriously now," Cortessis said, adding that the rates of testicular cancer have been rising inexplicably over the past century.

But even if the drug is in fact to blame for some of that increase, the danger isn't overwhelming. According to the American Cancer Society, a man's lifetime risk of getting testicular cancer is about one in 270. And because effective treatment is available, the risk of dying from the disease is just one in 5,000.

So far, little is known about what causes testicular cancer. Cortessis said undescended testicles - in which the testes remain in the abdomen beyond the age of a year - are a risk factor and that both pesticide and hormone exposure has been associated with the tumors.

Cortessis and her colleagues used data from 163 young men who had been diagnosed with testicular cancer and nearly 300 men in a comparison group without the disease. Both groups had been interviewed about their health and drug use between 1987 and 1994.

Among the cancer-ridden men, 81 percent had used marijuana at some point, whereas that was the case for 76 percent of the comparison group.

By contrast, cocaine use was linked to a smaller risk of the tumors. This is important, Cortessis said, because it signals that men who have been diagnosed with cancer aren't just more honest about their drug use, thereby creating a spurious link between marijuana and cancer.

She said it's possible that cocaine damages the cells in the testicles, including cancer cells. And, she stressed, "the take-home message is not that cocaine is helpful against testis cancer."

CONSISTENT FINDINGS

It's not entirely clear how marijuana would influence men's cancer risk, but Cortessis said developing testicles may somehow respond to the drug's main active ingredient.

Dr. Carl van Walraven from the University of Ottawa in Canada, who has studied testicular cancer, called the new study "interesting" but noted a number of limitations.

For instance, he told Reuters Health by email, it didn't find an increased risk among men with higher marijuana use and it was relatively small.

Cortessis, whose findings appear in the journal Cancer, highlighted the consistent results from all the studies so far.

"It is hard to imagine a scenario whereby it is due to chance and I can't think of a systematic bias that would cause this," she said. "I will feel very confident that this is cause and effect once we have worked out the biology."

SOURCE: bit.ly/UEE61v Cancer, online September 10, 2012.

• A Permanent Economic Emergency