OBJECTIVES: Previous evidence has shown that using Angiotensin Converting Enzymes (ACEIs) or Angiotensin Receptor Blockers (ARBs) results in decreased morbidity among patients with chronic heart failure (CHF). However, the benefits earned from medications depend on the amount of medications supplied and used. This study aims to determine the effects of medication supplies on healthcare costs and hospitalizations in CHF patients receiving ACEIs or ARBs. METHODS: We retrospectively examined an electronic patient database in a provincial hospital located in the north of Thailand. All patients with an ICD-10 of I-50.0 (CHF) receiving either ACEIs or ARBs from January to December, 2003 were included. Their medication supplies were assessed using Medication Possession Ratio (MPR) method during the study period following the index date. The association between medication supply (appropriate: MPR 0.8-1.20, oversupply: MPR>1.20, undersupply: MPR <0.8) and all-cause rehospitalizations within 1 year was determined using Cox-proportional hazard model. Total direct healthcare cost was compared between all groups using multiple linear regressions. All analyses were adjusted for propensity score (PS) and other variables including age, sex, prior health care use, and insurance status. RESULTS: Among 1012 CHF patients, 389 received ACEIs or ARBs. The mean age was 66 years with 56% female. Forty-two percent were undersupplied, while 56% and 2% were appropriately supplied and oversupplied respectively. The adjusted hazard ratios of under and oversupply for all-cause rehospitalizations were 1.21(95% CI; 0.79 -1.86) and 3.90 (95%CI;0.89-17.06). Comparing with the group appropriately supplied, total direct medical cost in the undersupplied group was significantly higher at 11,027 baht (95%CI; 325 - 21,727), while non-significant trend in the oversupply group was observed. CONCLUSIONS: Under medication supply is significantly associated with higher healthcare cost in patients with chronic heart failure.