Stevens-Johnson syndrome and toxic epidermal necrolysis are severity variants of the same disease which differs from erythema multiforme.

A new classification, based on the pattern and distribution of cutaneous lesions, separates erythema multiforme major from Stevens-Johnson syndrome. A retrospective re-classification of 76 cases supported the validity of that separation by demonstrating differing causes and pathology. Another prospective international case-control study found differing demographic characteristics and risk factors between erythema multiforme major on the one hand and Stevens-Johnson syndrome or toxic epidermal necrolysis on the other. Erythema multiforme major was mainly related to Herpes virus infection, while Stevens-Johnson syndrome and toxic epidermal necrolysis were associated with drug reactions.