Arsenic is a deadly poison. Even very small amounts can result in poisoning, and sometimes death.
So it comes as a surprise to hear that the Chinese have been giving it to leukaemia patients, and with some success. The therapy has been tested on 12 patients in America who were given doses of arsenic trioxide, ranging from between 0.06 to 0.2 mg per kilogram of body weight until the leukaemia was eliminated from the bone marrow. Complete remission lasted from 12 to 39 days, and adverse reactions were light, such as rash and fatigue (N Eng J Med 1998; 339: 1341-8).
To be fair, arsenic has been used in medicine for centuries and it never killed off the victims in the Ealing comedy Arsenic and Old Lace: it was the strychnine and cyanide.