A Unique Interaction of Methotrexate and Nitrofurantoin Resulting in Irreversible Pulmonary Fibrosis.

Pulmonary toxicity is the most well-known severe complication related to both methotrexate and nitrofurantoin, which can present as acute, subacute, and chronic. Rheumatoid arthritis is also known to cause pulmonary disease if left untreated. In this report, we present a unique case of a 94-year-old female being treated with methotrexate for several years and then treated with nitrofurantoin in the setting of rheumatoid arthritis and chronic urinary tract infections, resulting in irreversible pulmonary fibrosis, which can further cause more susceptibility to infections and pneumonia. Drug-drug interactions are common in polypharmacy and a patient's history should be analyzed thoroughly before prescribing any new medication that can cause more harm to the patient than good.