RT Journal Article SR Electronic T1 Patients with polyethylene glycol allergy can experience immediate-type hypersensitivity reactions after exposure to analog substances JF BMJ Case Reports JO BMJ Case Reports FD BMJ Publishing Group Ltd SP e254040 DO 10.1136/bcr-2022-254040 VO 16 IS 8 A1 Yoshimoto, Yosuke A1 Tezuka, Junichiro YR 2023 UL http://casereports.bmj.com/content/16/8/e254040.abstract AB Polyethylene glycol (PEG) allergy has been recently observed after COVID-19 mRNA vaccination. We present a case of a patient with a history of two hospitalisations for unexplained recurrence of immediate-type hypersensitivity reactions and anaphylaxis who was diagnosed with PEG allergy in early childhood. Subsequently, he was instructed to avoid using PEG-containing daily necessities and drugs. However, in middle childhood, he presented with immediate-type hypersensitivity reactions after taking PEG-free antibiotics. The prick test was positive for the whole drug but negative for its active ingredient. PEG can cross-react with compounds with a C–C–O skeleton as analogue substances; accordingly, the presence of a substance with a similar skeleton in the additive may have been the causative factor. Our findings indicate that patients with PEG allergy may experience immediate-type hypersensitivity reactions to analogue substances.