IntroductionEculizumab is a fully humanized monoclonal blocking antibody to complement protein C5 that inhibits cleavage to C5a and C5b, thus preventing terminal complement complex C5b-9 and formation of the membrane attack complex.1 Eculizumab was FDA approved in 2020 for the treatment of neuromyelitis optica (NMO) after it was shown to be effective in reducing relapse frequency in highly clinically active, aquaporin-4 immunoglobulin G (AQP4-IgG)-positive NMO.2 Commonly reported side effects (>10%) include upper respiratory infections and headache. A life-threatening desquamating rash and hyperammonemia following administration of eculizumab for paroxysmal nocturnal hemoglobinuria (PNH) has been reported.3 Cutaneous adverse drug reactions can range from self-limited cutaneous eruptions such as maculopapular exanthema to severe cutaneous drug reactions. Severe cutaneous adverse reactions are rare, potentially life-threatening, and T-cell mediated hypersensitivity reactions.4 Certain drugs can induce autoantibodies rather than cause an autoantibody-associated disease. The information available suggests eculizumab is unlikely to do this.5 We report a patient with refractory NMO who developed a cutaneous drug reaction following intravenous eculizumab administration. This information will be useful to clinicians, given the expanding clinical uses of eculizumab in diseases such as atypical hemolytic uremic syndrome (aHUS), PNH, and myasthenia gravis.6-8 Eculizumab has also been used for lupus nephritis-associated thrombotic microangiopathy in systemic lupus erythematosus patients.9