Methotrexate is the drug with the highest long-term continuation rate in rheumatoid arthritis patients. However, toxicity is the main reason for methotrexate withdrawal. Most adverse effects are mild abnormalities, such as digestive symptoms, stomatitis, elevations in transaminase levels, and moderate decreases in peripheral blood cell counts. Potentially life-threatening effects include hypersensitivity pneumonitis and pancytopenia. Cirrhosis is less common than in patients with psoriasis. Opportunistic infections and Epstein-Barr virus-related lymphomas have been reported. Neurological disorders, cutaneous reactions and renal lesions have been ascribed to low-dose methotrexate. Prior renal dysfunction and concomitant administration of a number of drugs, including cotrimoxazole, have been shown to increase methotrexate toxicity. However, susceptibility to the toxic effects of methotrexate varies widely across individuals. The effectiveness of folate supplementation in preventing methotrexate toxicity remains controversial.