BackgroundMetformin is associated with a reduced risk of some cancers but its effect on prostate cancer is unclear. Some studies suggest only Asians derive this benefit. Therefore, we undertook a systematic review with particular attention to ethnicity.MethodsMedline, Embase, Scopus, Web of Science, and EBM Reviews were searched from inception to 2015. Two reviewers identified and abstracted articles. Studies were pooled using random effects model and stratified by Western- vs Asian-based populations.ResultsWe identified 482 studies; 26 underwent full review. Of Western-based studies (n = 23), two were randomized trials and 21 were observational studies. All Asian-based studies (n = 3) were observational. There were 1,572,307 patients, 1,171,643 Western vs 400,664 Asian. Across all studies there was no association between metformin and prostate cancer (RR: 1.01, 95%CI: 0.86-1.18, I2: 97%), with similar findings in Western-based trials (RR: 1.38, 95%CI: 0.72-2.64 I2: 15%) and observational studies (RR: 1.03 95%CI: 0.94-1.13, I2: 88%). Asian-based studies suggested a non-significant reduction (RR: 0.75, 95%CI: 0.42-1.34, I2: 90%), although these results were highly influenced by one study of almost 400,000 patients (propensity-adjusted RR: 0.47 95%CI 0.45-0.49). Removing this influential study yielded an estimate more congruent with Western-based studies (RR: 0.98 95%CI:0.71-1.36, I2: 0%).ConclusionThere is likely no association between metformin and risk of prostate cancer, in either Western-based or Asian-based populations after removing a highly influential Asian-based study.Electronic supplementary materialThe online version of this article (10.1186/s12885-017-3934-9) contains supplementary material, which is available to authorized users.
BACKGROUND - A variety of methods are used to define exposure in pharmacoepidemiologic studies. Although each method has known biases, the relative effect of these biases on an observed association has not been fully examined.
OBJECTIVE - To explore the influence of different exposure definitions on estimates, using the association between metformin and all-cause mortality as a proto-typical model.
METHODS - New users of oral anti-hyperglycemic drugs were identified using administrative health databases from Alberta, Canada between 1998 and 2010. Drug exposure was described using definitions that are commonly used in observational studies. All analyses included the same covariates of age, gender, and a comorbidity score, and subjects not exposed to metformin served as the reference group. The measure of association was assessed using a Cox Proportional Hazards model for cohort studies and conditional logistic regression for case-control studies.
RESULTS – We identified 64,293 new oral anti-hyperglycemic drugs users; mean age 68.9 years, 33,131 (52%) males, and 24,745 (39%) deaths during a mean follow-up of 6 years. In adjusted models, the association between metformin and mortality ranged from 0.23 (95% CI 0.22-0.25) to 0.92 (95% CI 0.88-0.95) reduction. Most metformin exposure definitions, however, provided estimates in the 0.6-0.8 reduction range, aligning with the results of previous observational studies.
CONCLUSIONS – The variety of exposure definitions tested in this analysis produced a wide range of associations between metformin and mortality risk. Therefore, pharmacoepidemiological studies should implement sensitivity analyses including at least two exposure definitions to provide more robust and potentially valid study estimates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.