The use of metformin in patients with prostate cancer and the risk of death.

Background: Given the conflicting results from observational studies, we assessed whether the use of metformin after a prostate cancer diagnosis is associated with a decreased risk of cancer-specific and all-cause mortality.

Methods: This study was conducted linking four databases from the United Kingdom. A cohort of men newly diagnosed with nonmetastatic prostate cancer with a history of treated type II diabetes, between April 1, 1998 and December 31, 2009, was followed until October 1, 2012. Nested case-control analyses were performed for cancer-specific mortality and all-cause mortality, in which exposure was defined as use of metformin during the time to risk-set. Conditional logistic regression was used to estimate adjusted rate ratios (RR) of each outcome with 95% confidence intervals (CI).

Results: The cohort consisted of 935 men with prostate cancer and a history of type II diabetes. After a mean follow-up of 3.7 years, 258 deaths occurred, including 112 from prostate cancer. Overall, the post-diagnostic use of metformin was not associated with a decreased risk of cancer-specific mortality (RR, 1.09; 95% CI, 0.51-2.33). In a secondary analysis, a cumulative duration ≥938 days was associated with an increased risk (RR, 3.20; 95% CI, 1.00-10.24). The post-diagnostic use of metformin was not associated with all-cause mortality (RR, 0.79; 95% CI, 0.50-1.23).

Conclusion: The use of metformin after a prostate cancer diagnosis was not associated with an overall decreased risk of cancer-specific and all-cause mortality.

Impact: The results of this study do not support a role for metformin in the prevention of prostate cancer outcomes.