The finding is potentially valuable, the study authors said, given the ongoing debate over legalising marijuana for medical purposes.
But one urologist not involved with the study was sceptical of the finding, and noted that nonsmokers weren't among the men included in the study.
How the study was done
For the study, the researchers compared the risk of bladder cancer in more than 83 000 men who smoked cigarettes only, marijuana (cannabis) only, or both substances. The investigators found that men who only smoked pot were the least likely to develop bladder cancer over the course of 11 years.
"Cannabis use only was associated with a 45% reduction in bladder cancer incidence, and tobacco use only was associated with a 52% increase in bladder cancer," said study author Dr Anil A. Thomas, a fellow in urology at Kaiser Permanente Medical Center in Los Angeles.
Smoking both tobacco and marijuana raised the risk of bladder cancer, but less so than for those who only smoked tobacco, Thomas found. He presented the findings Monday at the American Urological Association annual meeting in San Diego.
Dr Karim Chamie, an assistant professor of urology at the David Geffen School of Medicine at the University of California, Los Angeles, took issue with the fact that every man in the study smoked something, so there was no comparison to men who did not smoke at all.
Difficulty in the study
"It's hard to judge a study when the reference group is not nonsmokers," Chamie said. And the number of bladder cancers diagnosed in the study seemed low, he added, especially because everyone smoked something and smoking tobacco is considered a leading risk factor for the disease.
"We know smoking increases your risk of bladder cancer by two- or threefold," Chamie noted.
Also, the men in the study weren't representative of the entire United States, Chamie said, noting all were insured residents of California.
Thomas said his research team wasn't looking at all age groups and was not collecting data on lifetime prevalence.
Thomas stressed that he's not condoning or recommending marijuana as a way to affect risk of bladder cancer. The value of the study may be to spur other research into new treatments for bladder cancer, he said.
The men in the study, aged 45 to 69, were patients at Kaiser Permanente in California. Thomas and his team evaluated data on their lifestyle habits, including tobacco and marijuana smoking, between 2002 and 2003.
The researchers cross-referenced the study data with medical records to see who was diagnosed with bladder cancer. They omitted men with a history of bladder cancer.
Overall, 41% of the men reported marijuana use, 57% said they used tobacco and 27% reported using both.
During 11 years of follow-up, 279 men - 0.3% - were diagnosed with bladder cancer. Eighty-nine pot smokers (0.3%) developed bladder cancer compared to 190 (0.4%) who did not smoke pot.
More frequent marijuana use - smoking pot more than 500 times - was associated with greater risk reduction than infrequent marijuana use - smoking once or twice, the researchers found.
Thomas said that while the study saw a link between smoking marijuana and lower bladder cancer risk, compared to tobacco smokers, it did not prove a cause-and-effect relationship. He couldn't explain the link, but speculated on the possible mechanism.
"The theory is that there are receptors in the bladder that are affected by cannabis," he said. The cannabinoids [compounds] in the marijuana may link with the cannabinoid receptors in the bladder and somehow protect against cell changes that can lead to cancer, he said.
A man's lifetime risk of bladder cancer is almost 4%, according to the American Cancer Society. This means about one in 26 men will develop bladder tumours.
To learn more about bladder cancer, visit the US National