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Background Pirfenidone and Nintedanib are antifibrotic medications licensed in the UK for Idopathetic Pulmonary Fibrosis (IPF), with Nintedanib also approved for Progressive Fibrosis Interstitial Lung Diseases (PFILD). However adverse drug reactions (ADRs) frequently lead to treatment discontinuation. This study aimed to identify baseline patient characteristics associated with tolerability of antifibrotic therapy.Method A retrospective analysis was conducted at a UK tertiary centre, reviewing all patients initiated on Pirfenidone and Nintedanib between May 2020 and April 2023. Tolerators were defined as patients who continued therapy for at least six months; non-tolerators discontinued within six months. Baseline demographics, pulmonary function tests, comorbidities, blood results and treatment initiation strategies were extracted. Comparative analyses were performed to explore associations between baseline characteristics and tolerability outcomes. ADRs were classified as any undesirable effects attributed to the antifibrotic medications, as recorded in follow up consultations. Dispensing records were cross-referenced to verify dosing history and treatment discontinuation.Results Among 260 patients included (72% IPF, 28% PFILD), 75% (n=195) were classified as tolerators. Tolerability was higher with Nintedanib than Pirfenidone (79.6% vs. 67.3%; p=0.027). Among tolerators, ADRs were common (59%) with no difference between nintedanib and pirfenidone. Age was the only baseline demographic associated with tolerability (mean age 72.4 vs. 74.8 years, p=0.44). Sex, body mass index, ethnicity, smoking status, and lung function (FVC and DLCO) showed no significant associations. Comorbidities including hypertension, diabetes, gastrointestinal disorders, anxiety and depression, and ischaemic heart disease were similarly distributed between groups. No significant associations were found between tolerability and baseline blood markers, or concurrent medications such as proton pump inhibitors, antiplatelet medications, immunomodulators or corticosteroids. Antifibrotic initiation strategy (standard vs. reduced dose) and prescriber type (consultant, specialist pharmacist, or nurse) did not impact tolerability.Abstract P26 Figure 1Correlation between patient characteristics, clinical features, initiation dosing on antifibrotic therapy tolerance. This table shows statistical association between baseline patient characteristics (age, gender, body mass index (BMI), ethnicity), clinical parameters (pulmonary function results, comorbidities, blood report), antifibrotic initiation strategies (dosing schedules, and initiating healthcare professional) with antifibrotic tolerability, defined as continuation of therapy for at least 6 months. Values represent averages, proportions or percentages corresponding to p-values to indicate significanceConclusion In this real-world cohort, antifibrotic therapy was tolerated by most patients, with better persistence observed in younger individuals and those on Nintedanib. Other baseline factors showed no clear association with tolerability.