There was also a remarkable decrease in plasma FGF21 levels after treatment with some drugs, such as exenatide, metformin, or mitiglinide, in new-onset T2DM patients (Samson et al., 2011; Wang et al., 2012; Fan et al., 2016; Hu et al., 2016). Here, FGF21 is linked to type 2 diabetes mellitus.