We present the case of an 80‐year‐old lady known to be sensitive to chlorocresol (4‐chloro‐3‐methyl phenol) who developed severe erythrodermic exfoliative dermatitis with atypical features 2 weeks after commencing subcutaneous insulin. All medications except insulin were stopped, without major improvement. It was noted that the insulin contained m‐cresol (m‐methyl phenol) so a parabens‐based insulin was substituted. There was a significant improvement in her clinical condition within 72 hr. Further patch and intradermal testing to the insulin and m‐cresol was planned, but she developed a nosocomial infection and died. We hypothesize that the adverse cutaneous reaction was a systemic manifestation of cresol sensitivity, given the rapid clinical resolution on changing insulins and the previously demonstrated sensitivity to chlorocresol, particularly as cross‐reactivity between different low molecular weight methyl phenols is documented. Local injection site reactions and systemic side‐effects including nausea, diarrhoea and vomiting have previously been reported with cresol‐containing insulins, although to our knowledge, this is the first reported case of a severe cutaneous reaction. It is important to be aware of m‐cresol as a potential allergen, as it is contained in most commercially available insulins.