Drug Allergy in Hospitalized Patients: Three Years of Consultation Experience in a Tertiary Care Setting

Background/Objectives: Drug hypersensitivity reactions (DHRs) are an important cause of morbidity in hospitalized patients, but their epidemiology and management in the inpatient setting are not well defined. Mislabeling of drug allergies may lead to inappropriate treatment and reduced antimicrobial stewardship. This study aimed to characterize the clinical profile, diagnostics, and management of inpatients referred for suspected drug allergy in a tertiary care hospital. Methods: We retrospectively reviewed all adult inpatients (≥18 years) at Luigi Sacco Hospital (Milan, Italy) who received allergology consultation between 1 June 2022 and 31 May 2025. Data on demographics, reaction type, culprit drugs, investigations, and management were collected. Immediate reaction severity was graded using the United States Drug Allergy Registry (USDAR) scale; delayed reactions were classified as severe cutaneous adverse reactions (SCARs) or non-SCARs. Logistic regression identified predictors of severity. Results: Among 35,438 admissions, 334 patients (0.9%) were evaluated; median age was 65 years, 51.2% were female, 67.4% had atopic comorbidities, and 55.1% reported prior drug allergy. Immediate reactions occurred in 49.1%, delayed in 43.7%. Cutaneous involvement was present in 86.8%, anaphylaxis in 6.6%, and SCARs in 3.9%. Antibiotics—particularly β-lactams—were most often implicated. In multivariate analysis, antibiotic exposure and older age were linked to more severe immediate reactions, while the absence of atopy predicted SCARs. Desensitization was successfully performed in 16.2% of patients. Conclusions: DHRs in inpatients are frequent and often involve high-risk drugs. Structured inpatient allergology services and an “allergy stewardship” approach may reduce DHR-related risks, support optimal therapy, and improve antimicrobial use strategies in tertiary care settings.