INS and type 2 diabetes mellitus: The aims of the study were (1) to identify intronic SNPs in genes encoding the OCT cluster that are associated with the occurrence of side effects during metformin use, and to replicate these findings in two cohorts of individuals with disturbances of insulin sensitivity—namely, T2DM and in women with PCOS on metformin medication; and (2) to investigate published metformin-related SNPs in relation to metformin side effects.