Methotrexate Pneumonitis After a Low-Dose Medication Error: A Case Report.

Methotrexate is recommended as the first choice of standard drug therapy following the diagnosis of rheumatoid arthritis. Pneumonitis related to methotrexate is a serious, unpredictable adverse event that may become life-threatening. We reported a case of a 68-year-old woman with rheumatoid arthritis that misunderstood the directions for use and took methotrexate daily, instead of weekly, leading to hepatic, hematological, and pulmonary toxicity.