Metformin does not prevent cancer in diabetic patients
Treatment with metformin does not appear to provide an additional benefit of reducing the incidence of cancers, excluding those of the prostate and pancreas, in patients with diabetes, as reported in a study.
Researchers looked at a cohort of 315,890 incident diabetic individuals aged 21–87 years, among whom 304,582 had no previous diagnosis of cancer. More than half of the population (n=172,948; 54.7 percent) took metformin at some time during their follow-up.
Weighted cumulative metformin exposure was evaluated in relation to cancer incidence using a time-dependent covariate Cox model, which was controlled for treatments with other glucose lowering medications, as well as age, sex, ethnic background, socioeconomic status, smoking (for bladder and lung cancer) and parity (for breast cancer). Metformin exposure in the year before cancer diagnosis was excluded in the analysis to minimize reverse causation of cancer on changes in medication.
Results showed no clear association between the use of metformin and all-sites cancer (excluding prostate and pancreas), and cancers of the colon, breast, lung and bladder. The hazard ratios with exposure to one defined daily dose of metformin over the previous 2–7 years were 0.98 (95 percent CI, 0.82–1.18) for all-sites cancer (excluding prostate and pancreas), 1.05 (0.67–1.63) for colon cancer, 0.98 (0.49–1.97) for bladder cancer, 1.02 (0.59–1.78) for lung cancer and 0.88 (0.56–1.39) for female breast cancer.
The study was limited by reliance on medication purchase data to identify medication use, relatively short study duration, and lack of data on confounding risk factors for cancer.