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Objective: To examine factors influencing medication expenditure for treating bipolar disorder by using the Laspeyres index. Materials and Methods: Retrospective utilization data were retrieved from a Thai hospital’s database. Changes in medication expenditure between 2008 and 2012 compared to between 2013 and 2017 were attributed to two factors, number of patient (Q) and cost per patient (P) for each 5-years. By measuring quantity as defined daily dose (DDD), changes in P comprised two sub factors, DDD per patient (q) and cost per DDD (p). Moreover, sub factor was weighted average with percentage of patient mix for q index and with percentage of product mix for p index. Results: Expenditure on medication in bipolar patients rose from 11.19 to 19.83 million Thai Baht (THB) per year between 2008 to 2012 and 2013 to 2017 (77.2%). With Laspeyres index, the total index of 1.77 was a result of three main factors, change in DDD per patient (q index at 1.40), weighted cost per DDD (p index at 1.16), and weighted number of patient (Q index at 1.09). The only factor that made a fall in expenditure was percentage of product mix, which meant more generic medications have been prescribed within 10 years. Conclusion: Over the last 10 years, only the impact of product mix has been a negative factor declining the total expenditure in bipolar disorder medication. Switching from brand to generic prescription may be the most suitable strategy to control medication expenditure in the authors’ setting. [ABSTRACT FROM AUTHOR]