New research says smoking pot may be less likely to cause bladder cancer than
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
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,"
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
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
To learn more about bladder cancer, visit the US National