Leflunomide is a medication used to treat rheumatoid arthritis. It works by suppressing the immune system, which can help reduce joint pain, swelling, and stiffness associated with the disease. It is available in tablet form and is taken orally. Leflunomide is a disease-modifying antirheumatic drug (DMARD) that can slow the progression of joint damage caused by rheumatoid arthritis. It is usually prescribed by a doctor and should be taken as directed. Common side effects of leflunomide include nausea, diarrhea, headache, and hair loss.