OBJECTIVE: Due to collective evidence of pilocarpine interfering with patients' activities and reducing compliance, it is considered a third line drug and reserved when the first and second line drugs have failed. Pilocarpine is, however, more commonly used in Thailand's general hospitals partly due to its availability as one of a two group A (generally available) topical drugs for treating glaucoma on the Thai national essential drug list. This study examines the effects of pilocarpine on health related quality of life (HRQOL) of Thai glaucoma patients. METHODS: Glaucoma patients were interviewed using the Thai Visual Function Questionnaire 28, consisting of 28 items in 11 domains, ranging from 0 to 100 with higher domain scores indicating better QOL. This questionnaire has good reliability, with Cronbach's alpha above 0.7 in all domains. The validity was established using visual acuity (VA) and visual field (VF). Multivariate analysis using linear regression was conducted. RESULTS: Of 367 patients recruited, 218 were female (59.4%), the mean age was 61 years (range 18-86). Patients using pilocarpine (N=67) had significantly lower visual function, education, and income than those without pilocarpine (N=242). Univariate analysis shows that patients using pilocarpine had significantly lower HRQOL scores in all domains. After adjusting for patients characteristics (age, sex), disease (VF, VA, duration of disease, other eye diseases), and treatment (surgery, number of drugs, frequency of administration), patients using pilocarpine showed significantly lower HRQOL scores in distance activities (beta = -13.03), social (beta = -13.72), mental (beta = -16.29), dependency (beta = -21.22), color vision (beta = -16.04), and peripheral vision domains (beta = -12.90). CONCLUSIONS: Pilocarpine was associated with a significant decline in HRQOL scores of up to 21 points. Adjustment of the national essential drug list to make more drugs avialable in group A for glaucoma treatment could be warranted in Thailand.