Empirical data of medication-related hospitalization are very limited. We aimed to investigate the associations between 12 high risk medication categories (diabetic agents, diuretics, nonsteroidal anti-inflammatory drugs (NSAIDs), anticoagulants, antiplatelets, antihypertensives, antiarrhythmics, anticonvulsants, antipsychotics, antidepressants, benzodiazepine (BZD)/Z-hypnotics, and narcotics) and unplanned hospitalizations. A population-based case-time-control study was performed using Taiwan Adverse drug events (ADEs) and their associated morbidity and mortality 1 represent a significant burden on the healthcare system 2 . Systemic reviews have estimated that approximately 5-10% of total hospital admissions are related to an ADE 3,4 . For the years 2004-2005 in the US, there were more than 700,000 ADE-related emergency department visits annually, and 1 of every 6 led to subsequent hospitalizations 5 . The annual cost of ADE in the US has been estimated at more than $ 136 billion 6 . In addition, the elderly are the most vulnerable to ADE-associated unplanned hospitalization 7 . For the elderly, the percentage of hospitalization attributed to ADE is estimated at 3.4-16.6% 8 . Since a great proportion of ADE-related hospitalizations are preventable 4 , efficient identification of ADE-related hospitalizations is therefore crucial to help highlight area which clinicians and policy-makers can put efforts in. Previous studies have adopted different approaches to identify ADE-related hospitalizations, such as analysis of spontaneous reporting data 9 , medical chart review 5,7,8,[10][11][12][13][14] , and screening diagnostic codes from electronic medical databases [15][16][17] . However, these methods are plagued with under-reporting issues and reporting bias. In addition, some of the methods are personnel-costly and time-consuming, resulting in studies with small sample sizes that cannot be generalized 10,[12][13][14] . Furthermore, it is difficult to interpret the results between different medication categories in existing studies, since most studies only provide descriptive data on