Azathioprine is an antimetabolite immunosuppressive agent that was a major factor in rapidly ushering in the early years of solid organ transplantation. Dermatologists have used this moderate-potency immunosuppressive agent most commonly for bullous dermatoses, such as the pemphigus and pemphigoid spectrums, as well as for leukocytoclastic vasculitis and adults with severe atopic dermatitis. The adverse effects of greatest concern have been hematologic toxic effects (eg, agranulocytosis and pancytopenia) along with lymphoma risk (primarily in patients who have undergone organ transplantation). In addition, liver toxic effects (with or without the presence of the drug hypersensitivity syndrome) and gastrointestinal (GI) tract symptoms, such as nausea and vomiting, can result in the cessation of azathioprine therapy in selected patients (Table).