Recreational use of marijuana puts men at a higher risk of developing the deadliest forms of testicular cancer, according a new study in the US.

Scientists from the University of Southern California (USC) say that the potential cancer-causing effects of marijuana on testicular cells should be considered not only in personal decisions regarding recreational drug use, but also when marijuana and its derivatives are used for therapeutic purposes in young male patients.

Testicular cancer is the most common cancer diagnosed in young men ages 15 to 45 years. The malignancy is becoming more common, and researchers suspect this is due to increasing exposure to unrecognised environmental causes.

To see if recreational drug use might play a role, the team of researchers looked at the history of recreational drug use in 163 young men diagnosed with testicular cancer and compared it with that of 292 healthy men of the same age and race/ethnicity.

The investigators found that men with a history of using marijuana were twice as likely to have subtypes of testicular cancer called non-seminoma and mixed germ cell tumors. These tumors usually occur in younger men and carry a somewhat worse prognosis than the seminoma subtype.

The study's findings confirm those from two previous reports on a potential link between marijuana use and testicular cancer.

"We do not know what marijuana triggers in the testis that may lead to carcinogenesis, although we speculate that it may be acting through the endocannabinoid system-the cellular network that responds to the active ingredient in marijuana-since this system has been shown to be important in the formation of sperm," said Victoria Cortessis, assistant professor of preventive medicine at the Keck School of Medicine of USC, Los Angeles.

The researchers also discovered that men with a history of using cocaine had a reduced risk of both subtypes of testicular cancer.

While it is unknown how cocaine may influence testicular cancer risk, the researchers suspect that the drug may kill sperm-producing germ cells since it has this effect on experimental animals.

The research was published in published in CANCER, a peer-reviewed journal of the American Cancer Society.