Men who smoke cannabis are at greater risk of testicular cancer, a new study suggests, but men who use cocaine may be at a lower risk from the disease.
Researchers have found a link between recreational cannabis use and an increased risk of developing subtypes of testicular cancer.
The findings say not only should marijuana users consider the risk in smoking the drug, but also doctors who administer it for therapeutic purposes in young male patients.
Testicular cancer is the most common cancer diagnosed in young men aged 15 to 45.
To see if recreational drug use might play a role, Doctor Victoria Cortessis, at the Keck School of Medicine at the University of Southern California 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 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 tumours.
These tumours 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 cannabis use and testicular cancer.
Dr Cortessis said: '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.'
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 authors suspect that the drug may kill sperm-producing germ cells since it has this effect on experimental animals.
Dr Cortessis added: 'If this is correct, then "prevention" would come at a high price. Although germ cells can not develop cancer if they are first destroyed, fertility would also be impaired.
'Since this is the first study in which an association between cocaine use and lower testis cancer risk is noted, additional epidemiological studies are needed to validate the results.'
The findings were published online in the journal Cancer.