This study aimed to investigate the prevalence of potential unnecessary injection (PUNI) and to identify factors associated with frequent PUNI uses. Population-based National Health Insurance claims data for outpatient health care settings during the six month period from July to December 2011 were retrospectively reviewed. Patients aged 18-80 without severe diseases and visited healthcare centers more than 25 times during study period were included. PUNI was defined as injection used where substitutable oral agents were available and where injection uses are not warranted. A total of 801,532 patients were included for this analysis. Among them, 29.0 % were frequent PUNI user defined as ≥10 PUNI used during the study period. In multivariate logistic regression results revealed significant associations between frequent PUNI use and several patient and healthcare utilization factors. Women than men, elderly than younger people, residents in rural areas than in big cities, and more frequent visitors to healthcare centers than less frequent visitors were more likely to be frequent PUNI users. Larger number of healthcare center utilized and higher out-of-pocket expense level showed significant lower risks of frequent PUNI uses. Identified factors associated with frequent PUNI use in this study could be the targets to develop programs for reducing injection overuse.