WASHINGTON (Reuters) - Marijuana use may increase the risk of developing testicular cancer, in particular a more aggressive form of the disease, according to a U.S. study published on Monday.

A cannabis leaf in a file photo. REUTERS/File

The study of 369 Seattle-area men ages 18 to 44 with testicular cancer and 979 men in the same age bracket without the disease found that current marijuana users were 70 percent more likely to develop it compared to nonusers.

The risk appeared to be highest among men who had reported smoking marijuana for at least 10 years, used it more than once a week or started using it before age 18, the researchers wrote in the journal Cancer.

Stephen Schwartz of the Fred Hutchinson Cancer Research Center in Seattle, one of the researchers, said the study was the first to explore marijuana’s possible association with testicular cancer.

“This is the first study to look at this question, and by itself is not definitive. And there’s a lot more research that would have to be done in order to be more confident that marijuana use really is important in a man’s risk of developing testicular cancer,” Schwartz said in a telephone interview.

The study found the increased risk appeared to be in the form called nonseminoma testicular cancer. It accounts for 40 percent of cases and can be more aggressive and more difficult to treat, Schwartz said.

Experts are unsure about the causes of testicular cancer, which often strikes men in their 20s and 30s. The disease is seen more commonly in men who have had an undescended testicle or have a family history of testicular cancer.

The disease usually responds well to treatment and has a five-year survival rate of about 96 percent, according to the American Cancer Society.

About 8,000 men in the United States are diagnosed with testicular cancer per year, and there are about 140,000 U.S. men alive who have survived the disease, the group said.

The researchers said they were not sure what it was about marijuana that may raise the risk. Chronic marijuana use also can have effects on the male reproductive system including decreased sperm quality, they said.