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Background: Nonsteroidal anti-inflammatory drugs (NSAIDS) are among the most common causes of drug hypersensitivity (HS) reactions. The diagnosis is based on a careful clinical history, and provocation tests are considered the gold standard for diagnosis. Skin tests have some value to study reactions to pyrazolones. Laboratory investigations are mostly used for research purposes. Different phenotypes have been described. Objective and Methods: Our aim was to describe the most common clinical manifestations of NSAID HS in a large population of adult patients, the drugs involved, the association with previously described risk factors, and the outcome of diagnostic procedures. The classification of reactions proposed by the European Academy of Allergy and Clinical Immunology (EAACI) Drug Allergy Interest Group was adopted. Results: Acetylsalicylic acid was the drug most often involved in reactions (34%), isolated cutaneous symptoms were the most reported (60%), and immediate reactions (58%) were the most common. There was an overall female predominance (64%) and 35% of the patients were atopic. HS to NSAIDs was confirmed in 21% of the patients. The most common phenotypes encountered among HS patients were NSAID-induced urticaria/angioedema and single-NSAID-induced urticaria/angioedema or anaphylaxis. Logistic regression analysis showed that gender and atopy were not significant risk factors for HS confirmation, but diagnosis depended on the number of previous reactions, the type of reaction, and the time interval between drug intake and reaction. Conclusion: Only 21% of suspected HS reactions were confirmed after diagnostic workup. Patients describing >1 previous reaction and suffering immediate reactions had a higher probability of a positive investigation. [ABSTRACT FROM AUTHOR]