LAUSR

5-Fluorouracil (Efudix) cream is established as a topical treatment for superficial malignant and premalignant skin lesions. Its method of action involves the irreversible binding of the pyrimidine analogue fluorouracil to thymidylate synthetase within a cell. This prevents the incorporation of uracil into nuclear RNA, which destroys abnormal cancer cells (https://dermnetnz.org/topics/5-fluorouracil-cream). The expected sequelae of its use involves the development of a marked inflammatory response. We present a case of a severe, disproportionate reaction to Efudix cream, secondary to contact allergy to the excipients. A 61-year-old man attended the cutaneous allergy clinic with a history of severe, florid, inflammatory and ulcerative skin reactions affecting the lower limbs at sites of application of Efudix cream. This had been used as directed, to treat areas of Bowen disease, at intervals between December 2019 and February 2021. Contact allergy to Efudix cream was suspected and patch testing was performed to the British Standard and Cosmetic series, as well as the excipients of Efudix cream, including stearyl alcohol, propylene glycol (PG), methylparahydroxybenzoate, propylparahydroxybenzoate and white soft paraffin. While the patch tests were applied in the department on day 0, subsequent appointments on days 2, 4 and 7 were performed virtually with photographs as the patient developed COVID-19 symptoms, with positive lateral flow and polymerase chain reaction tests. He was patch test positive on days 4 and 7 to stearyl alcohol and propylene glycol, both being excipients of Efudix cream. A review of our database over a period of 17 years revealed 53 other cases with positive patch test to PG (n = 53/8000;0.66%), none of which were attributable to the use of Efudix cream, and only six cases of a positive patch test to stearyl alcohol (n = 6/8000;0.075%), of which one was attributable to the use of Efudix cream. Allergic contact dermatitis to Efudix cream and its excipients stearyl alcohol and propylene glycol is rare, although it has previously been described in the literature with the earliest reports in 1992, and the most recent being 15 years ago [Meijer B, de Waardvan der Spek F. Allergic contact dermatitis because of topical use of 5-fluorouracil (Efudix cream). Contact Dermatitis 2007;57: 58-60]. This case adds to the existing literature and is a reminder to clinicians that, although inflammation is expected with the use of Efudix cream, severe or disproportionate reactions should raise suspicion of possible contact allergy. Furthermore, this case highlights the challenges of patch testing in the current COVID-19 climate and highlights the importance of teledermatology as a novel option for assessment in cutaneous allergy services facing these conditions.