Brimonidine propanone impurity is a process- or degradation-related species encountered during brimonidine manufacture that can affect assay accuracy, stability and safety assessments. Often formed via propanone-mediated side reactions or oxidative cleavage of the parent scaffold, this impurity is identified and characterized using orthogonal analytical approaches such as reversed-phase HPLC with photodiode array detection, LC-MS for exact mass confirmation and 1H/13C NMR for structural elucidation. Robust quantitation supports impurity profiling and risk assessment in line with ICH expectations. Effective control measures include optimized synthetic conditions, targeted purification steps and validated analytical methods with defined acceptance criteria to protect product quality.