Methotrexate is considered the first-line disease-modifying antirheumatic drug (DMARD) for rheumatoid arthritis. It works by suppressing the overactive immune response that drives joint inflammation, helping to reduce pain, swelling, and long-term joint damage. Typically taken once a week (by pill or injection), it can take 4–6 weeks before noticeable improvement. Because it affects rapidly dividing cells, side effects may include nausea, fatigue, mouth sores, and rarely liver or lung toxicity—so patients usually take folic acid supplements and undergo regular blood monitoring. Despite these risks, methotrexate is highly effective and often combined with other medications for optimal control of RA.