Men who have smoked marijuana are twice as likely to be diagnosed with testicular cancer than men who have never done so, a small new study finds.

Published Monday morning in Cancer, the journal of the American Cancer Society, the research compared 163 men who had been diagnosed with testicular cancer to 292 men (of similar ages and race/ethnicity) who had not. The men were asked about their health history, education and use of recreational drugs such as marijuana and cocaine, among other things.

The study, conducted by researchers at the University of Southern California, found no difference in risk of seminoma, the most readily treatable of the forms of testicular cancer, between men who had ever used marijuana and those who had not. But those who had used marijuana were more than twice as likely as those who hadn’t to have had non-seminoma and mixed germ cell tumors, which typically affect younger men and can be harder to treat.

The study cites data showing testicular cancer to be the most common malignancy among men ages 15 to 45. According to the National Cancer Institute (NCI), most testicular cancers start in cells (called germ cells) that make sperm and are thus called testicular germ cell tumors. About 8,950 men will be diagnosed with testicular cancer in 2012, according to the NCI; about 360 of them will die. Treatments vary according to the form of cancer and the extent to which it has spread through the body; they include surgery, radiation and chemotherapy.

The apparent connection between cancer risk and marijuana use found in the study was not exactly cut and dried: Former users of marijuana turned out to be at greater risk of testicular cancer than current users, and infrequent users at greater risk than frequent users. Go figure.

Another head-scratcher: After controlling for other factors, the research found that men who had used cocaine actually were less likely to have had testicular cancer than men who hadn’t.

The study notes that more research is needed to determine how and why marijuana use might contribute to development of testicular cancer. But the authors observe that the increased incidence of that cancer in recent decades has coincided with an increase in marijuana use.