Pethidine hydrochloride (PH) and fentanyl citrate (FC) are opioid receptor agonists commonly used to treat pain clinically. PH and FC have been reported to have a high potential for pseudoallergic effects, but the underlying mechanism has not been well studied. MRGPRX2 is a novel atypical opioid receptor that is mainly expressed in human mast cells and considered to mediate drug-induced pseudoallergic reactions. This study aimed to investigate the allergy effect of these two opioid receptor agonists and the possible association of MRGPRX2 with this response.