World Health Organization (WHO) established ATC system of drug classification. All drugs are grouped in five levels (anatomical main group, therapeutic subgroup, pharmacological subgroup, chemical subgroup and chemical substance). Alterations in ATC classification are made only by experts in WHO centre in Oslo, Norway. Changes are made in assigned new international generic names (INN), in ATC levels likes in changes of Defined Daily Doses (DDD) once a year. Changes are made following current scientific articles and international pharmacopeas, as well as the guideline.