Laird Harrison

May 10, 2013

SAN DIEGO, California — Smoking marijuana might decrease the smoker's risk for bladder cancer, a new study shows.

Retrospectively analyzing a large database of patients, researchers at Kaiser Permanente in California found that patients who reported cannabis use were 45% less likely to be diagnosed with bladder cancer than patients who did not smoke at all.

"It's very exciting because bladder cancer is hard to treat," said Anil Thomas, MD, a urologist at the Southern California Permanente Medical Group. But he cautioned that the study does not prove that smoking marijuana prevents bladder cancer, and more research is needed to explore the connection.

Dr. Thomas presented the study here at the American Urological Association 2013 Annual Scientific Meeting.

"We know that tobacco smoking is the best established risk factor for bladder cancer," Dr. Thomas told reporters attending a news conference. "But to date, there are no epidemiologic studies accurately characterizing the association between cannabis use and bladder cancer."

To fill that gap, Dr. Thomas and his colleagues analyzed a survey of 82,050 men from Northern and Southern California Kaiser Permanente, a health maintenance organization, conducted in 2002 and 2003.

The participants were aged 45 to 69 years.

In all, 41% reported using cannabis, 57% reported using tobacco, and 27% reported using both.

Distribution of Respondents by Extent of Cannabis Consumption

1–2 times 3–10 times 11–99 times 100–499 times More than 499 times
7% 9% 11% 7% 7%


Over the subsequent 11 years, more patients who reported no cannabis use than who reported use developed incident bladder tumors (0.4% vs 0.3%). The difference was statistically significant (P = .048).

However, patients who smoked only tobacco had an increased risk for bladder cancer, and those who smoked both tobacco and marijuana had about the same risk as those who smoked neither.

This might explain why previous studies have not uncovered any protective effects of marijuana, Dr. Thomas said. To see this effect, it's necessary to separate out the marijuana smokers who do not smoke tobacco.

Discussion moderator Kevin McVary, MD, chair of urology at Southern Illinois University in Springfield, asked if there were similar databases that could be used to verify these results. Dr. Thomas said he is not aware of any.

Dr. Thomas told Medscape Medical News that he first got interested in exploring this topic while doing some laboratory work in which he exposed prostate cancer cells and bladder cancer cells to cannabis.

"The prostate cancer cells did not show an effect and the bladder cancer cells were devastated," he said.

Some other research has suggested that cannabis might kill other types of cancer cells as well, he said. "I don't think the full mechanism is known."

Dr. Thomas and Dr. McVary have disclosed no relevant financial relationships.

American Urological Association (AUA) 2013 Annual Scientific Meeting. Presented May 6, 2013.