In recent years, researchers have evaluated the clinical efficacy and safety of Leflunomide (LEF) and other medications for the treatment of rheumatoid arthritis (RA). The objective of this work was chemical-quantum calculus and determination of the interaction type of LEF and collagen and its influence on arthritis. We obtained the information about the collagen of The National Center for Biotechnology Information (NCBI). It was obtained the formula of the LEF on the page of PubChem. The quantum calculations we carry out with the hyperchem software. These calculations were made in other articles already published by us. We concluded that 13 interactions of 40 calculated are significant. These 13 interactions show that only two of them are oxidant and the other 11 are reductive.