Formaldehyde Allergy Presenting with Anaphylaxis Due to Prolonged Immediate-Type Reaction.

Formaldehyde is a low-molecular-weight chemical used as a disinfectant and in some dental materials. Although formaldehyde is a well-known cause of allergic contact dermatitis, systemic allergic reactions related to dental treatment are rare and may be overlooked, particularly when symptoms are delayed or overlap with pre-existing allergic diseases. Here we present the case of a 36-year-old woman with a history of allergic rhinoconjunctivitis, atopic bronchial asthma, and eosinophilic gastroenteritis who presented with recurrent systemic allergic symptoms after root canal treatment. Eight years earlier, she had experienced nausea and diarrhea during endodontic therapy, which were attributed to eosinophilic gastroenteritis. During subsequent dental procedures, she developed delayed-onset abdominal pain, watery diarrhea, facial edema, generalized urticaria, dyspnea, and impaired consciousness. She was diagnosed with anaphylaxis and successfully treated with epinephrine. Extensive allergological evaluations, including oral drug challenge tests and skin prick and intracutaneous tests for aspirin, antibiotics, local anesthetics, and multiple dental materials, were negative. In contrast, skin prick testing with formaldehyde-containing dental agents was positive, and serum formaldehyde-specific IgE antibodies were detected, leading to the diagnosis of formaldehyde-induced anaphylaxis. This case highlights formaldehyde-containing dental materials as an under-recognized cause of anaphylaxis. Delayed symptom onset and overlapping manifestations with underlying allergic diseases may contribute to diagnostic delay. Awareness of formaldehyde allergy is essential in patients with unexplained or recurrent allergic reactions after dental treatment.