DPP4 and diabetes mellitus: Longitudinal cohort study using insurance claims data from 1/1/2012 to 12/31/2016 to identify patients with a first prescription, after metformin, of a second‐line diabetes drug (eg sulphonylurea, DPP‐4 inhibitor, thiazolidinedione, SGLT‐2 inhibitor or GLP‐1 receptor agonist) and to estimate their adherence to that drug class.