Despite, or possibly because of, its long-known toxicity, arsenic-containing potions and drugs have a history in medicine and quackery that continues into the 21st century.  Starting in the early 19th century and continuing into the 20th century, Fowler's solution, a toxic concoction of sodium arsenite, was sold. The organoarsenic compound Salvarsan was the first synthetic chemotherapeutic agent, discovered by Paul Ehrlich. The treatment, however, led to many problems causing long lasting health complications.  Around 1943 it was finally superseded by penicillin.
The related drug Melarsoprol is still in use against late-state African trypanosomiasis (sleeping sickness), despite its high toxicity and possibly fatal side effects.
Arsenic trioxide (AsO) inhibits cell growth and induces apoptosis (programmed cell death) in certain types of cancer cells, which are normally immortal and can multiply without limit. In combination with all-trans retinoic acid, it is FDA-approved as first-line treatment for promyelocytic leukemia.