In findings presented last week at the American Urological Association's annual conference, researchers announced that the conclusion of an 11-year study has found a strong association between frequent marijuana use and a significantly reduced bladder cancer risk.
The study, which has not yet undergone peer-review and was not published in any established journal, looked at the cancer risk of more than 83,000 men who smoke cigarettes, marijuana or both.
The researchers found that men who smoke cigarettes multiply their risk of bladder cancer, while men who smoke only marijuana actually lower their risk. Men who smoke both still had an elevated risk of bladder cancer, but it was lower than those who just smoked tobacco.
"Cannabis use only was associated with a 45 percent reduction in bladder cancer incidence, and tobacco use only was associated with a 52 percent increase in bladder cancer," study author Dr. Anil A. Thomas says.
More amazing still: study participants who used marijuana more than 500 times a year saw even lower bladder cancer risks than those who only used marijuana occasionally. A total of 41 percent of the men studied said they smoked marijuana, while 57 percent smoked tobacco and 27 percent smoked both.
Critics pointed out that the study could have been more thorough if it included a control group of non-smokers and filtered for other lifestyle habits.
The link between lower cancer risks and higher marijuana usage rates is still unexplained, but Thomas speculated that human bladders may have cannabinoid receptors that react to the non-psychoactive components of marijuana, protecting cells against dangerous mutations that can cause cancer.
Although another recent study found that men who used marijuana in their adolescence and then quit experience a slightly higher risk of testicular cancer, those who continued using the substance into adulthood did not show that same risk. Less recently, a 2007 study in the scientific journal Molecular Cancer Therapeutics found that a compound in marijuana called CBD can actually slow down and stop gene activity related to breast cancer.
For these and many other reasons, the American Medical Association voted in 2009 to recommend that lawmakers reclassify marijuana to allow for more scientific research into its potential medical uses. Congress, the White House and the courts, however, have refused to act.