Teenagers who smoke pot regularly may slightly increase their risk of developing the most aggressive type of testicular cancer compared with nonsmokers, according to a new study.

In Monday's online issue of the journal Cancer, epidemiologist Janet Daling and her colleagues at the Fred Hutchinson Cancer Research Center in Seattle interviewed 370 men aged 18 to 44 with testicular cancer about their history of marijuana use, and compared them with nearly 1,000 randomly selected healthy control subjects.

After accounting for other factors such as smoking tobacco, drinking alcohol and family history, using marijuana at least regularly or starting in adolescence was considered a significant risk factor for nonseminoma — a fast-growing testicular cancer that tends to strike between ages 20 and 35

The rare type of cancer is diagnosed in about 800 men in Canada every year, and accounts for about 40 per cent of all cases of testicular cancer.

"The risk was more than doubled if you were a frequent smoker of marijuana," Daling said.

It seemed to be particularly important if pot was smoked before the age of 18, the researchers found.

Among the cancer group, about 73 per cent said they had smoked pot at some point compared with 68 per cent in the other group.

The study does not prove that smoking marijuana causes cancer, and relied on self-reports of marijuana use.

But Daling said it's intriguing given that the incidence of testicular cancer has climbed in tandem with increasing use of marijuana since the 1950s.

Even if further research also points to an association, the risk would be small, said Dr. David Bell, a urologist with Capital Health in Halifax.

"This is a very rare malignancy, seen in about one per 100,000 men per year," said Bell. "One has to really question the significance of this."

The researchers did not find a link between marijuana and the more common form of testicular cancer.

It's unclear what causes testicular cancer, but marijuana is known to have some hormonal effects within the body, Bell said.

Until further research clarifies marijuana's effect, Daling advised that young men not smoke it, saying they are taking a chance on their future health.

The findings aren't worrying for Joel, a 17-year-old who has been smoking pot for more than two years.

"I don't think just 'cause they put another study out that I'll change my habits or how much I smoke cannabis."

Joel said that the relaxation he gains from smoking pot is too beneficial to give up.

The research was funded by the National Cancer Institute, the National Institute on Drug Abuse and the Hutchinson Center.