Hypertension (HT) is among the most prevalent chronic diseases in Thailand which leads to a considerable amount of healthcare expenditure. Owing to its high costs, concerns about efficiency of HT treatments have been raised, especially in the context of the Thai healthcare system where 3 main health insurances are publicly available: Civil Servants’ Medical Benefit Scheme (CSMBS); Social Security Scheme (SSS); and Universal Coverage (UC). Previous research has revealed the costs and health outcomes of HT treatment; however, little is known about the comparative cost efficiency of treatment across health insurances. The purposes of this study are two-fold: 1) to perform the cost-outcome analysis of HT treatment in uncomplicated HT patients across health insurances; and 2) to explore factors associated with the rising costs of HT treatment.  Data of uncomplicated HT patients who were treated at the out-patient (OP) department at Suratthani Central Hospital in 2014 were retrieved from electronic medical records. Direct medical costs of HT treatment were estimated from resources used in the OP care of HT, including the unit cost of an OP visit, and also the costs of laboratory tests, anti-hypertensive medications (essential drugs (ED) and non-essential drugs (NED)), and other HT-related treatments. The costs of HT treatment were approximated from 4 different perspectives: societal, patients’, healthcare payers’, and providers’, with base year 2014. A treatment outcome was a patient having well-controlled blood pressure (BP), defined as having systolic BP<140 mmHg and diastolic BP<90 mmHg. Chi-square and ANOVA were used in the statistical analysis, with a significance level of 0.05. Multiple linear regression analysis was employed to assess the attributes associated with the costs of HT. To evaluate the elements affecting the use of NED, multiple logistic regression analysis was applied.  A total of 4,353 patients were identified: 2,247 entitled to UC, 149 to SSS, and 1,957 to CSMBS. Statistics indicated that the proportions of patients with well-controlled BP were not significantly different across health insurances. From all perspectives, the costs of HT treatment were much higher in CSMBS beneficiaries, compared with those in the other insurance schemes. Comparing the 3 health insurances, the CERs of HT treatment are highest in CSMBS beneficiaries. Results demonstrated that the increased costs of treatment are associated with the use of NED, and that the likelihood of using NED increases in patients with CSMBS insurance in relation to those with UC and SSS.  Inefficiency of HT treatment appears in CSMBS beneficiaries. The higher costs of HT treatment are attributed, in part, to the use of NED. While no such clear evidence reveals advantages of NED to reduce blood pressure over ED, NED are relatively much more expensive than ED. As physicians are responsible for the choice of medical treatment in hypertensive patients, a hypothesis proposed in this study to explain physicians’ selection criteria for the use of NED, is the awareness of cost burden. That is, physicians tend to avoid the use of NED when the burden of the additional costs is either the patients’ or the hospitals’, as is the case with the UC and SSS beneficiaries, respectively. In contrast, the use of NED increases when the public insurance bears the incurred costs from NED prescription, as seen in CSMBS cases.  Future research, testing this hypothesis using a qualitative study, is recommended. Furthermore, as a reimbursement mechanism could be the key to reducing the overuse of NED, future research should explore the impact of a change in the reimbursement system, from fee-for-service to capitation, on the drug costs for CSMBS patients.