BACKGROUND Excipients are widely used in pharmaceuticals, detergents, food, and drink because of their properties of low toxicity and hypoallergenicity. The excipient carboxymethylcellulose is used extensively as a thickener in foods such as baked goods, ice cream, gluten free, and reduced fat products, where it may be labeled as e-number E466. However, excipients can rarely cause type 1 hypersensitivity reactions. Several publications have described systemic allergy following carboxymethylcellulose exposure in pharmaceuticals, particularly systemic corticosteroids. Furthermore, there is one reported case in the literature of anaphylaxis following food containing carboxymethylcellulose. METHODS We identify a case of anaphylaxis in a 45-year-old atopic Caucasian woman on receiving an injectable suspension of the corticosteroid triamcinolone acetonide containing carboxymethylcellulose, and subsequent allergic symptoms on reexposure to carboxymethylcellulose in a commercial drink. Diagnosis of carboxymethylcellulose excipient allergy was confirmed through skin prick testing using Celluvisc carmellose 0.5% eye drops, which contain carboxymethylcellulose as the active ingredient. CONCLUSIONS This case highlights the importance of identifying excipients such as carboxymethylcellulose as causes of allergy, to reduce burden of further hypersensitivity reactions, not just to drugs but to other consumables.