This was a cross-sectional survey research. The questionnaires were sent along with an accompanying letter by mail to 1,380 hospitals through out Thailand during December 2009 to January 2010. The questionnaire was designed to gather data in the area of personnel affiliation, the LASA drug list generic/brand name and the form of drug, including tablets/capsules, injectable drugs, liquid preparations, external use preparations and chemotherapy agents, as well as, a medication error categorization index (category A-I) for each LASA pair, according to the National Coordinating Council for Medication Error, Reporting and Prevention