The multivariate analysis showed an association between the FOXP3 rs3761548-GT genotype (GT vs. TT/GG: OR = 3.86, 95% CI = 1.17–13.92, p = 0.031) and the risk of having more than one adverse effect, adjusted for MTX administration route (both: OR = 15.92, 95% CI = 4.60–64.78, p < 0.001; subcutaneous: OR = 4.07, 95% CI = 1.26–14.43, p = 0.022) and developing psoriatic arthritis (OR = 4.28, 95% CI = 1.48–13.45, p = 0.009) (all these values are shown in Table 4). The gene discussed is FOXP3; the disease is psoriatic arthritis.