The main findings in this study showed that (1) the accumulated dose of MTX and PDN favor the increase of Bacteroides fragilis, (2) the changes in proportion of Lactobacillus/Porphyromonas gingivalis are associated with the increase serum levels of IL-17A and with the clinical activity of the RA, and (3) IFABP2 serum levels are positively associated with TNF-α levels and with secondary non-response to csDMARD in RA. Here, IL17A is linked to rheumatoid arthritis.