Methotrexate (MTX) is an inhibitor of folic acid metabolism as well as purine metabolism and T-cell activation.96 Its successful use in treating the arthritis and myositis of PM/DM as an adjunctive agent when steroids have failed112,113 is well established. No trials of MTX specifically for MA-ILD exist. Despite the absence of pulmonary-specific evidence and due to its relatively low side-effect profile, MTX has become accepted in the treatment of MA-ILD. Particular care must be taken in treating MA-ILD with MTX given the agent’s known association with idiosyncratic drug-related hypersensitivity pneumonitis.114 Using literature from rheumatoid arthritis cohorts, it can be estimated to affect approximately 0.5% of those who take the drug,115 with another review further classifying the risk as one event per 35.4 patient-years of therapy.116 In the rheumatoid arthritis population, it has been suggested that pneumonitis is more common in patients who are diabetic, have hypoalbuminemia, or are > 60 years of age, or, notably, in those in whom preexisting lung disease was noted.117 When this occurs, often early in the course of therapy,116 it can be difficult to distinguish symptoms as being related to the drug (using the major and minor criteria developed by Searles and McKendry)118 or a manifestation of the MA-ILD itself without a lung biopsy to obtain histopathology. Both the drug toxicity and a worsening of the ILD can present with interstitial infiltrates, increased dyspnea or cough, and a decline in pulmonary function, including a drop in the Dlco. Given this diagnostic dilemma, stopping the drug and switching immunomodulatory agents is prudent should pulmonary symptoms, PFT results, or CT scan findings worsen after starting this agent.