Marijuana users had a small but statistically lower risk of bladder cancer compared with people who smoked tobacco or did not smoke.
Marijuana users had a small but statistically lower risk of bladder cancer compared with people who smoked tobacco or did not smoke, a large retrospective cohort study showed.
The overall cohort had a bladder cancer risk of <1%. Nonetheless, cannabis (marijuana) users had an incidence of 0.3% compared with 0.4% among nonusers. The difference represented a 45% reduction in the hazard ratio (HR) for bladder cancer.
The magnitude of risk reduction appeared to increase with the frequency of cannabis use, Anil A. Thomas, MD, reported at the American Urological Association meeting in San Diego, California, in May.
“The full effects of cannabis use are not known, and Kaiser Permanente does not promote nor condone the use of cannabis,” said Thomas, an endourology fellow at Kaiser Permanente Los Angeles Medical Center. “However, our findings demonstrated a reduced incidence of bladder cancer among cannabis users, which could lead to further investigational studies to determine if cannabinoids, the active components of cannabis, have biologic activity against bladder cancer. Future research may eventually lead to the development of new targeted therapies for patients with bladder cancer.”
Bladder cancer is the fourth most common cancer in men, and tobacco use is a recognized risk factor for developing the disease. Whether that risk extends to cannabis use had not been examined in large epidemiologic studies. To address the issue, Thomas and colleagues analyzed records from the California Men’s Health Study (CMHS), a multiethnic cohort study that focuses primarily on prostate cancer, but also on the etiology of other cancers and non-cancerous conditions.
Hazard Ratio, as Compared with Nonsmokers
The bladder cancer study involved 84,170 men from the Northern and Southern Kaiser Permanente health regions participating in the CMHS. After excluding 680 men with a history of bladder cancer, investigators analyzed data for the remaining participants.
According to self-reported information provided by the participants, a total of 71% of the men were smokers (41% cannabis, 57% tobacco and, included in those numbers, 27% both substances). Meanwhile, 29% used neither cannabis nor tobacco. During follow-up for as long as 11 years, 279 study participants (0.3%) developed bladder cancer. Cannabis users accounted for 89 (0.3%) cancers and nonusers (including tobacco users) for 190 cancers (0.4%, P <.0001).
An analysis adjusted for age race/ethnicity, and body mass index showed that exclusive tobacco use was associated with a bladder cancer HR of 1.52 compared with nonusers (P <.007). In contrast, exclusive cannabis use was associated with a reduced risk of bladder cancer (HR=0.55; P =.0484). Men who reported use of tobacco and cannabis had a bladder cancer HR of 1.28, which was not statistically different from that of nonusers (P =.1480).
In another adjusted analysis, men who reported using cannabis once or twice had no reduction in bladder cancer risk (HR=1.07; P =.760), whereas men who reported use in excess of 500 times had a bladder HR of .66, which still did not achieve statistical significance (P =.175).
“While a cause-and-effect relationship has not been established, cannabis use may be inversely associated with bladder cancer risk in this population,” Thomas said. “Additional research is needed to determine if there is indeed a biological effect of cannabis to prevent or delay the development of bladder cancer.”
Thomas A, Wallner L, Quinn V, Slezak J, Chien G, Jacobsen S. Association between cannabis use and risk of bladder cancer: Results from the California Men’s Health Study. Presented at: the American Urological Association Annual Meeting; May 4-8, 2013; San Diego, CA. Abstract 1290.