The stability of four formulations of pilocarpine oral solution containing glycerol, hydroxybenzoate preservatives and lemon spirit was studied on storage for 90 days at 4, 25 and 38C using a stability‐indicating high performance liquid chromatography assay. The solutions were prepared from pilocarpine hydrochloride powder (0.2 and 3mg/ml) in citrate buffer pH 5.5, or from eyedrops (3mg/ml) in citrate buffer pH 5.5 or phosphate buffer pH 7.5. Significant decomposition of pilocarpine occurred in the pH 7.5 formulation at all three temperatures. The data were adequately described by a first order model. Degradation in the pH 5.5 formulations was not significant over 90 days at 4C or 60 days at 25C. Decomposition at 38C was significant and adequately described by a zero order model. The lower 95 per cent confidence limits for 10 per cent decomposition were 36 days for the eye‐drop formulation and 27 and 32 days for the 0.2 and 3mg/ml powder formulations respectively. Pilocarpine oral solution prepared from powder or eye‐drops in pH 5.5 buffer is chemically stable for 60 or 90 days at 25C or 4C, respectively, and provides a stable dosage
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