A method for the synthesis of a diaminophenothiazinium compound of the following formula: comprising (a) purifying a corresponding reduced compound which is N-acylated at the heterocyclic nitrogen; (b) deacylating the purified compound to provide the corresponding reduced compound; and (c) oxidizing the reduced compound to provide the diaminophenothiazinium compound. Optional purification may performed, for example, after deacylating and after oxidizing. The method provides high purity diaminophenothiazinium compounds which are suitable for pharmaceutical and related therapeutic uses. Such uses include inactivating pathogens, the treatment of the infectious diseases, and for the treatment of diseases of protein aggregation, such as tauopathies, including Alzheimer's disease.