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Background and Importance According to the literature, a 5% and 20% acetylcysteine (Hidonac ) eye drop compounding was prepared using a solution of artificial tears, containing 1.4% polyvinyl alcohol and 0.005% benzalkonium chloride (Liquifilm tears ), in low-density polyethylene (LDPE) dropper bottles for the treatment of dry keratoconjunctivitis. To evaluate stability, a 30-day galenic validation was performed, storing the eye drops under refrigeration (2–8°C). 1 Aim and Objectives Optimise and study the stability through galenic validation of 5% and 20% acetylcysteine eye drops formulated with a solution of artificial tears in LDPE dropper bottles.Material and Methods A 5% acetylcysteine eye drop solution was prepared using a solution of artificial tears, and a 20% acetylcysteine eye drop solution. All samples were prepared in a horizontal laminar flow cabinet, following the Good-Practice-Guidelines for sterile drug preparation. A 30-day galenic validation was performed, monitoring clarity, color, pH, osmolality, and microbiological stability on days 0, 1, 2, 7, 15, 22, and 30. Three units were tested per sampling point, in accordance with European Pharmacopoeia.Results The 5% acetylcysteine solution showed pH values between 6.83–7.32 (maximum deviation of 6.70% compared to day 0) and osmolality values of 786–839 mOsm/kg (maximum deviation of 6.74% compared to day 0). All samples presented a transparent and homogeneous appearence with no visible particles, except on the last day of the galenic validation, when a precipitate was observed that did not redisperse after shaking. No microbiological growth was detected throughout the galenic validation period.The 20% acetylcysteine solution showed pH values between 6.48–6.84 (maximum deviation of 5.27% compared to day 0). Osmolality was not measured because the equipment available in our hospital cannot measure such high values (theoretical osmolarity: 3800mOsm/L).On day 22, a precipitate was observed that did not redisperse after shaking. No microbiological growth was detected throughout the galenic validation period.Conclusion and Relevance Although both concentrations remained within the ocular pH tolerance range(3.5–10.5), precipitates were observed during galenic validation. Therefore, the 14-day validity period was determinated.Galenic validation is essential to ensure the properties of a compounding, as well as to detect stability issues.References and/or Acknowledgements 1. Anaizi N, et al. Stability of acetylcysteine in an extemporaneously compounded ophthalmic solution. American Journal of Health-System Pharmacy 54(5):549–553. https://doi.org/10.1093/ajhp/54.5.549Conflict of Interest No conflict of interest