Stevens-Johnson syndrome (SJS) is a severe and life-threatening condition. It is thought to be a hypersensitivity complex affecting the skin and the mucous membranes.

There is agreement in the medical literature that Stevens-Johnson syndrome can be considered a milder form of toxic epidermal necrolysis (TEN). However, there is debate whether it falls on a spectrum of disease that includes erythema multiforme; many consider erythema multiforme (EM) to be unrelated to SJS and TEN.

Some classify SJS as a severe expression of erythema multiforme, and it occasionally referred to as erythema multiforme major.