OBJECTIVES: METHODS: The prevalence of ADRs:A cross-sectional descriptive study. Data was collected from electronic database in Vajira Hospital during 1 January 2012-31 December 2013 from inpatient and outpatient departments.Those patients had experienced with those ADRs from other hospitals or been diagnosed for ADRs by physicians or pharmacists at Vajira Hospital.Costs of treatment for serious and fatal ADRs (baht per event) were direct medical costs.Costs included drug costs which were obtained from the median price from the Drug and Medical Supply Information Center, Ministry of Public Health.,cost of admissions to hospital and laboratory cost, which were obtained from the unit cost of the standard cost for HTA.The Costs were evaluated from the resources being used in medical records and verify by physicians. RESULTS: There were 737 identified ADRs events which consisted of ADRs type A (42events,6%) and ADRs Type B (695 events, 94%). Amlodipine was responsible for the most common ADRs type A (n=7,17%) for swollen feet while antimicrobial agents,ceftriaxone was responsible for the most common ADRs type B (n= 69, 10%).Majority of manifestation of drug allergies were skin systems (n=615, 83%).Serious and Fatal ADRs were showed for 42 events (6%),which antimicrobial agents were responsible for the most common causes (ceftriaxone n= 4) followed by NSAIDs (Ibuprofen n= 3)and anticonvulsants (Phenytoin n=4).Direct medical cost for the treatment of Serious and Fatal ADRs was about 41,000 baht per visit.  CONCLUSIONS: This study provided information to develop the medication safety policies, which some drugs should be monitored for severe allergic reactions.To increase patient safety and reduce costs arising from the treatment of those severe allergic reactions,the pharmacists should  counsel and educate patients to the level that those patients can be able to observe themselves.