Described are methods of synthesizing and purifying certain 3,7-diamino-phenothiazin-5-ium compounds (referred to herein as “diaminophenothiazinium compounds”) including Methylhioninium Chloride (MTC) (also known as Methylene Blue). In one embodiment, the method comprises the steps of, in order: a thiosulfonic acid formation (TSAF); an oxidative coupling (OC); and a ring closure (RC). Also described are resulting compounds and compositions comprising them (e.g., tablets, capsules) for use in methods of medical treatment and diagnosis, etc., for example, for tauopathies, or Alzheimer's disease (AD).