AIM: To determine initial medication adherence in newly diagnosed glaucoma patients treated with anti-glaucoma drugs. METHODS: This retrospective and observational study included all patients diagnosed with glaucoma in the Primary Health Care units in Portugal during the years 2012 and 2013, which in consequence received a first prescription for anti-glaucoma drugs. Data was collected from electronic prescribing records of the primary care units and from pharmacy claims records. Initiation of glaucoma treatment and early discontinuation were measured, and the combination of (non)-initiation and early discontinuation accounted for initial medication (non)-adherence. RESULTS: A total of 3548 new glaucoma patients (40.1% male; 59.9% female) were included. The 1133 (31.9%) patients were initially classified as non-users, since there was no pharmacy claim found for their first prescription for glaucoma treatment. Additionally, 277 (11.5%) patients early discontinued their treatment, acquiring only their first prescription. Overall, the initial medication non-adherence rate was 39.7% since 1410 patients either didn’t initiate treatment or discontinued it early. CONCLUSION: This study, reveals a major opportunity to improve glaucoma treatment and its control, since a large proportion of patients fail to engage with their prescribed therapy, which implies that implementation of individual or group strategies that enable patients with glaucoma to correctly perform their treatment is still needed.