Even though the above studies suggested P. copri as a key microbe associated with RA, a study on patients receiving treatment for RA found no such association:35 Neither the abundance of P. copri nor the family Prevotellaceae was significantly different between patients with RA and healthy controls.35 Methotrexate (MTX) is one of the main therapeutic agents used for the treatment of RA, and its mechanism of action depends on the inhibition of dihydrofolate reductase (DHF). Here, DHFR is linked to rheumatoid arthritis.