Internet photos of swimming phenomenon Michael Phelps, 23, possibly indulging in an Olympian lungful of marijuana smoke scandalized many and may have caused him to lose face and the faith of some of his fans -- not to mention a lucrative sponsorship from Kellogg.
But according to new research, if Phelps makes a habit of smoking up, he could stand to loose more than a fan base.
For the first time, researchers have linked frequent marijuana use to an increased risk of testicular cancer, according to an article published today in the journal Cancer.
"There's been very little research done on marijuana use and its association with cancer risk," said Janet Daling, the senior author of the article and an epidemiologist and member of the Public Health Sciences Division at the Fred Hutchinson Cancer Research Center (FHCRC) in Seattle. "This is the first time this association has been shown."
Using a population-based, case-control study, Daling surveyed 369 men diagnosed with testicular cancer and 979 healthy men between ages 18-44 about their history with marijuana use.
After controlling for family history and lifestyle factors, including alcohol and tobacco use, which could also be associated with testicular cancer, the researchers found that being a marijuana smoker was associated with a 70 percent increased risk of testicular cancer.
Participants who used marijuana once a week or more or who had long-term exposure to marijuana since adolescence were at twice the risk for testicular cancer than those who never used marijuana.
In addition, the association between frequent marijuana use and testicular cancer was more pronounced among men who had nonseminomas, a more aggressive form of testicular cancer, than those who had seminomas. Nonseminomas also tend to strike in younger men.
Although this research has the most implications for habitual users, it goes against commonly held beliefs that marijuana is one of the more benign recreational drugs, a potential concern given the number of people who have tried marijuana in their lives.
Testicular cancer is the most common cancer in white American males ages 15-34. Between 2001 and 2005, nearly half of all testicular cancer diagnoses were in men between ages 20-34, according to the National Cancer Institute.
Established risk factors include family history, undescended testes, and abnormal testicular growth. However, with about 8,000 cases diagnosed each year, testicular cancer is one of the rarer types of cancer.
But some have mixed feelings about the report.
"It is both surprising and not surprising," said Dr. Omer Kucuk, an expert in cancer and nutrition and a professor of hematology and medical oncology at the Winship Cancer Institute at Emory University in Atlanta. Kucuk was not associated with the research.
Kucuk pointed out that while this was the first paper amid limited research on marijuana and cancer to show a causal effect, there is evidence that marijuana does affect parts of both the male and female reproductive systems, including hormones and sperm quality.
In addition, Stephen Schwartz, co-author of the study, epidemiologist and member of the Public Health Sciences Division at the FHCRC, indicated that there may be room for improving the quality of the data gathered.
"We're reliant on them telling us what they did," Schwartz said. "We have to be a little skeptical of the accuracy of the reporting."
But the results showing that men with the aggressive, nonseminomal testicular cancer were more frequent drug users than those with seminomal cancer made Daling believe that patients were telling the truth overall, or were at least being consistent in their answers.
"If, when you're doing a cancer study, you might suspect that the cancer cases might be more honest when answering a question that is not routine," Daling said. "If that was the case, you'd see it across both histology groups. But we saw it only in nonseminomas, which indicates it's not just recall bias."
More substantial research on marijuana use being linked to increased risk for lung, head and neck cancers does exist, although it is not definitive. Kucuk said that trying to uncover the mechanism of how marijuana might be associated with cancer -- whether it could be hormonal or receptor-related -- could help focus future studies.
"Research like this is not meant for people to change their habits or ideas," Schwartz said. "It is one study out of many more that need to be done."
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