OBJECTIVES: A previous study in Thailand found that over possession of medication was not uncommon. As its financial burden has not been evaluated, this study is conducted to determine the prevalence of over possession and its financial loss. METHODS: We retrospectively examined the electronic patient database in a tertiary-care, university-affiliated hospital located in the north of Thailand. All patients receiving one of the top five high total expenditure medications (atorvastatin 20 mg, clopidogrel 75 mg, valsartan 160 mg, rivastigmine 200 mg, and rosiglitazone 8 mg) from January 2005 to December 2005 were included in the study. We determined the possession of medication using Gap-incorporated Medication Possession Ratio (Gap-MPR) method during the period of 1 year following the index date. The prevalence of over possession is defined as the proportion of those with Gap-MPR value greater than 1 over the total number of patients receiving the medication. We estimated the financial loss only in patients with Gap-MPR greater than or equal to 1.2. Descriptive statistics were used for statistical analyses. RESULTS: A total of 2227 subjects were initially identified but only 1893 patients were included in the analysis. The average age was 65.2, while the majority was female (56%). The prevalence of over possession (Gap-MPR >1) ranged from 23.2% (rivastigmine) to 62.8% (rosiglitazone). The annual financial loss due to over possession was estimated as US$1,251 (THB39,751), US$1,163 (THB36,992), US$1,692 (THB53,813), US$4,656 (THB148,051) and US$1,823 (THB57,985) for rosiglitazone, valsartan, rivastigmine, atorvastatin and clopidogrel, respectively. The total financial consequence was US$10,585 (THB336,592) or 1.22% of total medication costs. CONCLUSIONS:The financial loss due to over possession is not small and can be a significant burden to hospital administrators. Development of measures to prevent the unnecessary over possession of medication is needed.