Arsenic trioxide (ATO) is used to treat acute promyelocytic leukemia in people who have not been helped by other types of chemotherapy or whose condition has improved but then worsened following treatment with other types of chemotherapy. Arsenic trioxide acts through activation of Jun N-terminal kinase (JNK), activator protein-1, and inhibition of dual-specificity phosphatases. Although the exact mechanisms under which ATO exerts its therapeutic effect in acute promyelocytic leukemia cancer cells are not well elucidated. It was shown that apoptotic mechanisms involved the induction of phosphatidylserine externalization, caspase-3 activation, and nucleosomal DNA fragmentation. Adverse reactions described are leukocytosis, nausea, vomiting, diarrhea, and abdominal pain, fatigue, edema, hyperglycemia, dyspnea, cough, rash or itching, headaches, and dizziness.