Fellas, you might want to think, well, twice about following Olympic swimmer Michael Phelps' lead. A study published today in the journal Cancer linked frequent marijuana use to the possibility of a slim increased risk of testicular cancer.

Researchers from Fred Hutchinson Cancer Research Center in Seattle found that about 72 percent of 369 men, ages 18 to 44, diagnosed with this type of cancer reported having smoked pot; those at greatest risk appeared to have started toking before they were 18 and/or were heavy users. But the scientists acknowledge the study did not prove a connection between pot and a heightened risk of the disease, which strikes about 8,000 men in the U.S. annually and has a high survival rate, according to the American Cancer Society. The percentage of healthy men who reported having smoked pot at least once–68 percent of a 979 randomly sampled group–is not much lower than the group who had already been diagnosed with testicular cancer.

The prevailing belief has been that a man's chances of developing testicular cancer is largely determined in the womb, as cells in the fetus are developing and those known as germ cells (which later develop into sperm cells) fail to mature properly. This work shows the possibility that marijuana use–an environmental factor–might also play a role. But researchers acknowledge that it does not prove a definitive link–and that there were weaknesses to the study, including that it was based on a relatively small group of men and relied on self-reported drug use, which can be iffy. 

Still, lead study author Janet Daling, an epidemiology professor at the Fred Hutchinson Cancer Research Center in Seattle, says that she got the idea of a possible association between pot and testicular cancer about a decade ago at a lecture that noted the presence of receptors sensitive to THC (tetrahydrocannabinol, the active ingredient in cannabis) in the testes in addition to the known cannabinoid receptors in the brain. (Since then, there have also been cannabinoid receptors found in the heart, uterus, spleen and other locations in the body.) The researchers propose that consuming a lot of THC that activates these sensitive receptors might cause them to forgo some of their normal communication of the body's own similar system, (called the endocannabinoid system) which has been associated with the suppression of tumor growth.

"We're not exactly sure what role the marijuana is playing," Daling says, but it has come out as a possible factor that warrants further investigation.

Len Lichtenfeld, deputy chief medical officer for the American Cancer Society, says that the study is a potentially promising clue, but that is by no means a firm conclusion. He notes that other lifestyle choices, such as diet, tobacco smoking and exercise, have already been shown to have an effect on the risk of other cancers, so this is another potential risk factor that men can consider. And to this point, better-accepted risk factors for testicular cancer in particular include family history, undescended or abnormal testes.  

Other studies have shown possible links between pot smoking and impotence and infertility. Oh, and the possibility of a three-month suspension from the men's swimming team.

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