Background <p>Dextromethorphan (DM) is a cough suppressant that is widely available in many prescribed and over-the-counter medications. Both immediate and delayed hypersensitivity reactions have been reported following the ingestion of DM. Anaphylaxis to DM is rare, with only three reported cases in the literature. Among these, skin prick testing for DM yielded a positive result in one case, a negative result in another, and was not performed in the third.</p> Case presentation <p>We present a rare case of anaphylaxis linked to DM, confirmed by skin testing. The patient experienced a severe allergic reaction after taking Vicks Dayquil Complete®, a common over-the-counter cold and flu medicine that contains acetaminophen, DM, phenylephrine hydrochloride, and guaifenesin. Among these, only DM triggered a positive response on skin testing.</p> Conclusion <p>This case highlights an uncommon allergy to a combination cold medicine, with skin testing identifying DM as the cause.</p>