However, other studies have presented contradictory results. Smiechowski et al used data on patients "newly-treated" with oral anti-diabetic medications from the United Kingdom Clinical Practice Research Datalink to conduct a nested case-control designed study. Researchers matched "incident cases of colorectal cancer occurring during follow-up" with a max of 10 controls. Adjusted rate ratios of colorectal cancer "associated with ever use, and cumulative duration of use of metformin" were then calculated.
Results showed that metformin use and duration of use was not linked to incidence of colorectal cancer among patients (RR: 0.93; 95% CI: 0.73-1.18). The authors conclude the study by recommending against the start of metformin randomized controlled trials for the prevention of colorectal cancer.
Smiechowski B, Azoulay L, Yin H, Pollak MN, Suissa S. The use of metformin and colorectal cancer incidence in patients with type 2 diabetes mellitus. Cancer Epidemiol Biomarkers Prev. 2013.