Described are methods of synthesis and/or purification of certain 3,7-diamino-phenothiazin-5-ium compounds (referred to herein as “diaminophenothiazinium compounds”) including Methylthioninium Chloride (MTC) (also known as Methylene Blue), the resulting (high purity) compounds, compositions comprising such compounds (e.g., tablets, capsules), and the use of such high purity compounds. The compounds are useful in inactivating pathogens, and methods of medical treatment, prophylaxis, and diagnosis of diseases including a tauopathy, related neurological and infectious diseases.