Tehran University of Medical Sciences Iranian Journal of Allergy, Asthma and Immunology 1735-1502 22 4 2023 09 03 409 412 Maria Pasali Allergy Unit “D. Kalogeromitros”, 2nd Department of Dermatology and Venereology, National and Kapodistrian University of Athens, ATTIKON University General Hospital, Athens, Greece Styliani Taka Allergy and Clinical Immunology Unit, 2nd Pediatric Clinic, National and Kapodistrian University of Athens, Athens, Greece 3 StArtBioP.C., Molecular Allergy Diagnostics, Athens, Greece Caterina Chliva Allergy Unit “D. Kalogeromitros”, 2nd Department of Dermatology and Venereology, National and Kapodistrian University of Athens, ATTIKON University General Hospital, Athens, Greece Alexandros Katoulis 2nd Dpt. of Dermatology and Venereology, National and Kapodistrian University of Athens, ATTIKON University General Hospital, Athens, Greece Michael Makris 2nd Department of Dermatology and Venereology, National and Kapodistrian University of Athens, ATTIKON University General Hospital, Athens, Greece 2022 05 04 2023 07 15 Polyethylene glycols (PEG) or macrogols are polymers of ethylene oxide widely used in drugs either as active substances or, more commonly, as excipients. We report a Caucasian 32-year-old woman with referred anaphylaxis almost instantly after oral intake of a macrogol-containing laxative. Despite an anaphylactic reaction, the patient showed negative results for both the skin test and specific IgE to monomer, while the basophil activation test and oral challenge were positive. The patient was later successfully vaccinated with a polysorbate 80-containing SARS-CoV-2 vaccine following an additional work-up. As a result, the inactive form of PEG cannot be fully diagnosed, and it is considered a “hidden” allergen. Allergens like polysorbates need special consideration due to their possible cross-reactivity by their specific derivatives. https://ijaai.tums.ac.ir/index.php/ijaai/article/view/3535 https://ijaai.tums.ac.ir/index.php/ijaai/article/download/3535/1977