A study that included veterans from Georgia and California found that those who had a higher use of marijuana were more likely to get bladder cancer.
The study is in the January issue of the journal Urology. Of the 52 men younger than 60 who had a form of bladder cancer, 46, or 88 percent, had used marijuana extensively compared with 72 of 104 men of similar age with no cancer, a rate of less than 70 percent.
The study grew out of what Medical College of Georgia professor of urology Dr. Martha K. Terris and her colleagues were seeing at the Augusta Department of Veterans Affairs Medical Centers and at a VA affiliated with Stanford University, where Terris worked previously.
"Basically, we had some patients that seemed exceptionally young to be getting bladder cancer," Terris said.
"One thing they seemed to all have in common was marijuana. It could have been just chance, it could have been that they were in their early 50s and at a VA hospital and therefore were in Vietnam, and very few people got through Vietnam without testing a little marijuana."
But even after factoring in other risk factors such as cigarette smoking, "the marijuana use in the young bladder cancer patients was significantly higher than in those that did not have bladder cancer," she said. Chemicals in marijuana, particularly tetrahydrocannabinol, or THC, are absorbed into the bloodstream from the smoke.
"But then it is filtered out of the bloodstream through the kidneys and actually concentrated in the urine. So these chemicals have even more effect in the urine than they would necessarily in the bloodstream."
And THC lasts five times longer in the urine than nicotine byproducts, which would increase the exposure of the bladder tissue, she said.
But THC has had a mixed history in cancer studies, and in some cases actually killed cancer cells, said Dr. Deborah Lewis, an MCG researcher who studies the receptors in the brain affected by that marijuana-like compound.
In one study, THC increased breast tumor growth by dampening the immune response; in another, it killed cancerous cells from the immune system, she said.
"I think there needs to be a lot more clinical and basic science work done on this issue of cancer" and those compounds, Lewis said. "Some studies show it helps, and some studies show it doesn't."
Allen St.Pierre, the executive director of the National Organization for the Reform of Marijuana Laws, said he can point to three or four studies in which marijuana compounds were found to have an anticancer effect.
He believes the problem might be combining it with smoking cigarettes, which many in the VA study did.
"It looks like when you separate marijuana out from the tobacco consumption, counterintuitive as it is, there are these sort of anticancer qualities to it," St.Pierre said.
"So this study seems to affirm that if you use tobacco and marijuana there is a greater likelihood you could have these problems."
Terris said cigarette smoking might play a role.
"There may be a combined effect," she said. While the medical marijuana debate rages in other states, the study could serve as a caution about its benefits.
"Certainly in patients who may be receiving chemotherapy for advanced bladder cancer, I think smoking marijuana for the side effects of that chemotherapy is inadvisable," Terris said.