A new study by researchers at Fred Hutchinson Cancer Research Centre has revealed that frequent and/or long-term marijuana use may significantly increase a man's risk of developing the most aggressive type of testicular cancer.

Researchers found that being a marijuana smoker at the time of diagnosis was linked to a 70 per cent increased risk of testicular cancer.

The risk was particularly elevated for those who used marijuana at least weekly and/or who had long-term exposure to the substance beginning in adolescence.

The results also suggested that the association with marijuana use might be limited to nonseminoma, a fast-growing testicular malignancy that tends to strike early, between ages 20 and 35, and accounts for about 40 percent of all testicular-cancer cases.

"Our study is not the first to suggest that some aspect of a man's lifestyle or environment is a risk factor for testicular cancer, but it is the first that has looked at marijuana use," said author Stephen M. Schwartz, M.P.H., PhD, an epidemiologist and member of the Public Health Sciences Division at the Hutchinson Centre.

For the study, the researchers interviewed 369 Seattle-Puget Sound-area men, ages 18 to 44, who had been diagnosed with testicular cancer about their history of marijuana use.

For comparison purposes they also assessed marijuana use among 979 randomly selected age and geography-matched healthy controls. (More than 90 percent of the cases and 80 percent of the controls in the study were Hispanic or non-Hispanic white men, due to the fact that testicular cancer is very rare in African-Americans, and because the Seattle-Puget Sound region has a relatively small African-American population.)

Study participants were also asked about other habits that may be correlated with marijuana use, including smoking and alcohol consumption.

Even after statistically controlling for these lifestyle factors, as well as other risk factors, such as first-degree family history of testicular cancer and a history of un-descended tests, marijuana use emerged as a significant, independent risk factor for testicular cancer.

The researchers emphasize that their results are not definitive, but rather open a door to more research questions.

"Our study is the first inkling that marijuana use may be associated with testicular cancer, and we still have a lot of unanswered questions," Schwartz said, such as why marijuana appears to be associated with only one type of testicular cancer.

"We need to conduct additional research to see whether the association can be observed in other populations, and whether measurement of molecular markers connected to the pathways through which marijuana could influence testicular cancer development helps clarify any association that exists," he said.