Leflunomide belongs to a new generation of biologic drugs that work by blocking the action of a protein called interleukin-1 (IL-1). Interleukin-1 is a cytokine, which is a small protein made by cells in the immune system and can trigger inflammation. By stopping IL-1 from being made, leflunomide helps to reduce both the signs and symptoms of rheumatoid arthritis and psoriatic arthritis.