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Background and Importance Bacterial eye infections at tertiary hospitals pose serious health challenges and require prompt treatment, often involving the preparation of fortified antibiotic eye drops. Since commercial products are not available, fortified antibiotic eye drops must be prepared in a hospital pharmacy unit, within a laminar-flow hood to maintain aseptic conditions.Aim and Objectives To evaluate and compare the stability of fortified gentamicin eye drops during four weeks of storage at room temperature and in the refrigerator in relation to the method of preparation and type of solvent.Material and Methods We prepared fortified gentamicin eye drops (1.36%) by using two methods: (i) adding a vial of intravenous gentamicin (80 mg/2 mL) to gentamicin eye drops 0.3% (5 mL) prepared in 1.92% boric acid solution and citrate buffer, and (ii) dissolving gentamicin sulphate in 1.92% boric acid solution and citrate buffer. We examined changes in pH, concentration and absorption values over four weeks related to the method of preparation, type of solvent and storage conditions.Results Our results demonstrated that the pH remained within stability limits (<10% change from the initial pH value) in all formulations, regardless of method of preparation, solvents or storage temperature. In fortified gentamicin eye drops prepared by dissolving gentamicin sulphate in 1.92% boric acid solution, gentamicin content decreased to almost half of its initial amount (57.2%) after the seventh day of storage, regardless of storage conditions. However, when prepared in citrate buffer, a slight loss of gentamicin content (up to 10%) was observed after 28 days of storage under ambient (25 °C) and refrigerated (2-8 °C) conditions. For formulations obtained by adding an intravenous gentamicin vial to ophthalmic gentamicin solution (0.3%) in 1.92% boric acid solution, the absorption values remained stable for 21 days. In contrast, the stability of formulations in citrate buffer appeared to be no more than 14 days.Conclusion and Relevance Significant differences were found between gentamicin ophthalmic formulations related to their method of preparation and type of solvent, while temperature had no effect on physicochemical characteristics. The optimal gentamicin ophthalmic formulation (1.36%) should be prepared by dissolving gentamicin sulphate in citrate buffer, as it has been proven to be the most stable.Conflict of Interest No conflict of interest