The diagnosis of Steven–Johnson syndrome (SJS), toxic epi- dermal necrolysis (TEN), and SJS/TEN overlap syndrome was proposed in 1993 according to the percentage of the body surface with skin sloughing: <10% in SJS, >30% in TEN, and 10–30% in SJS/TEN [1]. In Japan, a spectrum of skin conditions with epidermal detachment and mucous membrane erosions is defined as acute-phase erythema exsudativum multiforme major (EEMM). Acute-phase EEMM includes SJS, TEN, and drug-induced hypersensitivity syndrome (DIHS) (Table 23.1), and SJS/TEN overlap syndrome is classified as TEN. According to a nationwide survey conducted between 2016 and 2018 involving 315 cases of SJS and 174 cases of TEN, the suspected drugs in SJS were antibiotics (19.2%), antipyretic analgesic anti-inflammatory drugs (18.1%), antiepileptics (7.2%), drugs for peptic ulcers (6.9%), and cardiovascular drugs (6.6%) in descending order. In TEN, the order was antibiotics (30%), antipyretic analgesic anti-inflammatory drugs (13.5%), cardiovascular drugs (11.0%), drugs for peptic ulcers (6.7%), and anticancer drugs (4.7%). Differential diagnosis is often difficult in a very early stage of the disease.