Purpose: To determine the association between cancer diagnosis and medication adherence in patients with glaucoma. Design: Cohort study. Methods: SETTING: Population-based. STUDY POPULATION: All patients with confirmed glaucoma in one or both eyes. All patients were aged 20 years or older. Patients who developed cancer within 2 year of a confirmed glaucoma diagnosis or who were dead within 2 year after a confirmed cancer diagnosis were excluded. INTERVENTION OR OBSERVATION PROCEDURE(S): Individuals without cancer but with a confirmed glaucoma diagnosis were followed until they received a confirmed cancer diagnosis. They were then matched with a group of patients with confirmed glaucoma who did not develop cancer during the study period. MAIN OUTCOME MEASURE(S): Secondary adherence measured using medication possession ratio (MPR) at 1-year, 2-year, and 2-year-average intervals. Results: For both patients with cancer and their matches, MPR was the highest when measured at 1-year intervals. MPR was 0.379 (95% CI: 0.370-0.388) for the cancer group and 0.313 (95% CI: 0.308-0.319) for the cancer-free group. MPR measured using 1-year intervals decreased by 17.4% after cancer diagnosis (P <.001). MPR measured using 2-year and 2-year-average intervals decreased by 10.4% (P <.001) and 9.21% (P <.001), respectively. Conclusions: Cancer diagnosis leads to lower medication adherence in patients with glaucoma. To improve medication adherence in patients with glaucoma who also have cancer, policies should directly target the burden associated with having cancer that tends to create barriers for medication refills, rather than targeting risk factors that are also applicable to patients with glaucoma but without cancer.