Ronac Mamtani, M.D., from the University of Pennsylvania in Philadelphia, and colleagues conducted a cohort study involving 87,600 patients with type 2 diabetes in The Health Improvement Network database.
The incidence of bladder cancer in new users of metformin and sulfonylureas was examined. Metformin or sulfonylurea use was treated as a time-dependent variable. The authors hoped to address the time-related biases that may have affected previous studies relating to the effect of metformin on cancer risk.
The researchers identified 196 incident bladder cancers in the metformin cohort and 66 in the sulfonylurea cohort. The risk of bladder cancer was not significantly decreased in association with metformin use (hazard ratio, 0.81; 95 percent confidence interval [CI], 0.60 to 1.09).
The correlation was not affected by sex (P for interaction = 0.20). There was also no reduction in bladder cancer risk with increased duration of metformin use relative to use of sulfonylureas (three to less than four years of use: 0.57 [95 percent CI, 0.25 to 1.34]; four to less than five years of use: 0.93 [95 percent CI, 0.30 to 2.85]; and five or more years of use: 1.18 [95 percent CI, 0.44 to 3.19]; P for trend = 0.26).
“Similar methods should be used to study other cancers that have previously been identified as potentially preventable with metformin,” the authors write.
Several authors disclosed financial ties to the pharmaceutical industry.