Arsenic is a metalloid whose inorganic compounds exhibit toxic properties when accumulated in the human body. This chemical element is very common in nature as part of metallic ores, from where it enters water, plants and the atmosphere. Despite the fact that arsenic compounds are traditionally associated with poisoning, each person consumes about 20-50 µg of arsenic with food every day. It is present in apples, tomatoes, red wine, chocolate, pork, chicken meat and flour. Seafood is particularly rich in arsenic. In food arsenic is found in its non-toxic form in the form of the organic compound arsenobetaine. Inorganic arsenic compounds (arsenites and arsenates) can cause clinically pronounced intoxication. Recently, a positive effect of arsenic trioxide has been noted in the treatment of acute promyelocytic leukaemia. Although therapeutic doses of the drug (0.07-0.17 mg/kg/day) are used, in some cases clinical effects of acute intoxication, primarily cardiac rhythm disturbances, are observed. It is most difficult to assess the response to arsenic trioxide administration in elderly patients, as well as in patients with hypokalaemia and hypomagnesaemia and concomitant heart disease. It is advisable to adjust the dose of arsenic trioxide to the concentration of this substance in the blood. Arsenic levels in the blood are also measured in case of complications due to treatment with this drug.