The advent of biol. drugs in the past decade has significantly improved the treatment of several dermatol. diseases.These are commonly attributed to the active ingredient; however, the real culprit is often not disclosed. Patch testing, skin prick testing (SPT), and intradermal testing (IDT) have focused not only on the active ingredient but also on key excipients, such as polysorbate 80-present in the vehicle of many biologics, including secukinumab.A 40-yr-old woman with a history of hidradenitis suppurativa, who developed a paradoxical palmo-plantar psoriasis due to adalimumab, presented with a 4-mo history of injectionsite reactions to secukinumab 300 mg (Cosentyx).From the first administration, she developed extensive, well-defined erythematous-edematous itchy hives at the injection site, which started within an hour of administration and spontaneously resolved within 6 h.However, SPT with polysorbate 80 at 1:100 and 1:1000 dilutions, along with IDT using the same dilutions of polysorbate 80, and undiluted Cosentyx, yielded pos. results at 60 min.With reactivity to such high dilutions, a diagnosis of immediate reaction type contact urticaria due to polysorbate 80 was established.The drug was maintained, as all available biologics contain polysorbates, with the administration of 2 mg of oral dexchlorpheniramine the night before and 1 h prior to drug injection.A second-generation anti-H1 was considered, but dexchlorpheniramine was maintained due to good tolerance and patient′s preferences. This anti-H1 therapy was partially effective, reducing the size and duration of the injection-site wheals, and preventing the appearance of generalised wheals.When facing hypersensitivity reactions to biologics, excipients should be carefully considered, especially in cases of unclear origin and atypical clin. presentation. Since stages 3 and 4 of contact urticaria syndrome involve extracutaneous manifestations and potentially fatal anaphylactoid reactions to polysorbate 80 have been documented it may be necessary to identify hypersensitivity to this ubiquitous component.