The invention relates to a novel process for preparing N-methylnaltrexone bromide, comprising at least the steps consisting in: (i)reacting N-methylnaltrexone methyl sulfate in aqueous solution with an alkaline agent chosen from the group constituted by sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, caesium carbonate, strontium carbonate and mixtures thereof, for a pH of the aqueous reaction medium of between 7 and 10, and then in (ii)reacting the product thus obtained with hydrobromic acid, which is added for a pH of the aqueous reaction medium of between 0.5 and 5, in order thus to obtain the N-methylnaltrexone bromide.本發明係關於一種製備溴化N－甲基納曲酮之新穎方法，其至少包含由以下各者組成之步驟：(i)使N－甲基納曲酮甲基硫酸鹽於水溶液中與選自由下列各物組成之群的鹼性試劑反應：碳酸鈉、碳酸鉀、碳酸鈣、碳酸鎂、碳酸銫、碳酸鍶及其混合物，使得水性反應介質之pH值在7與10之間，且接著(ii)使由此獲得之產物與氫溴酸反應，添加該氫溴酸使水性反應介質之pH值在0.5與5之間，以由此獲得溴化N－甲基納曲酮。
