Influence of metformin intake on the risk of bladder cancer in type 2 diabetes patients.

Objective: The aim of this study was to look at the influence of metformin intake and duration, on urinary bladder cancer (UBC) risk, with sulfonylurea (SU) only users as control using a new user design (inception cohort).

Methods: We conducted a retrospective cohort study using data from the UK Clinical Practice Research Datalink (CPRD) including all patients with at least one prescription of oral anti-diabetic drugs (ADD) and/or insulin. The risk of UBC in different groups of ADD users (metformin alone (one), metformin in combination (two) with other ADD medication (glinides, glitazones, DPP-4-inhibitors, SUs, insulin or more than one combination), all metformin users (1 + 2) was compared with SU only users using Cox proportional hazards models. The estimates were adjusted for age, gender, smoking status, BMI and diabetes duration.

Results: The inception cohort included 165,398 participants of whom 132,960 were metformin users and 32,438 were SU only users. During a mean follow-up time of more than 5 years 693 patients developed UBC, 124 of the control group and 461 of the all metformin users. There was no association between metformin use and UBC risk (HR = 1.12, 95% CI 0.90, 1.40) compared with SU only users, even after adjustment for diabetes duration (HR = 1.13, 95% CI 0.90, 1.40). We found a pattern of decreasing risk of UBC with increasing duration of metformin intake, which was statistically not significant.

Conclusions: Metformin has no influence on the risk of UBC compared with SU in type 2 diabetes patients using a new user design.