Stability Assessment of Methadone Hydrochloride Oral Solution for Dispensing in Drug Addiction Centers: A Physicochemical and Microbiological Study.

Objective: The objective of this study was to determine the physicochemical and microbiological stability of a 0.5% aqueous oral solution of methadone for dispensing to Drug Dependency Centers in the Region of Murcia.

Materials and methods: Six different batches of 5 L methadone 0.5% solution were prepared in purified water. The samples were stored at controlled room temperature (<30°C), three protected from light and three not protected from light. Physicochemical analysis (concentration and pH) and microbiological analysis were performed on days 0, 15, 30, and 60 after preparation.

Results: The average concentration variation for samples protected from light was 98.4 (RSD 0.01%), and 100.97 (RSD 0.003%) for those not protected from light. pH values remained <6.5 throughout the study, increasing slightly over time. Organoleptic characteristics remained unchanged during the study period. Microbial growth was not observed in any of the samples analyzed, meeting acceptance criteria for oral liquid magistral formulations.

Conclusion: The 0.5% methadone solution showed chemical, physical, and microbiological stability for 60 days after preparation. These results have allowed extending the expiration date of the preparation and optimizing the dispensing circuit of methadone to Drug Dependency Centers.