Carboxymethylcellulose (CMC) is a derivative of cellulose found in many food products, pharmaceuticals and cosmetics. Allergy to CMC in parenteral corticosteroid preparations leading to anaphylaxis is rare, but has previously been reported. We report a case of a 52-year-old woman with prurigo nodularis of Hyde, who reacted with anaphylaxis after intradermal injection of Kenalog 40 mg/ml. Allergy testing showed a positive skin prick test for CMC and the patient was advised to avoid future parenteral exposure to CMC. This case highlights the need to examine excipients in severe cases of drug allergy.