This invention pertains generally to the field of phenothiazine compounds, and more particularly to certain stably reduced phenothiazine compounds, specifically, certain 3,7 diamino-10H-phenothiazine (DAPTZ) compounds of the following formula wherein: each of R1 and R9 is independently selected from: —H; C1-4alkyl; C2-4alkenyl; and halogenated C1-4alkyl; each of R3NA and R3NB is independently selected from: —H; C1-4alkyl; C2-4alkenyl; and halogenated C1-4alkyl; each of R7NA and R7NB is independently selected from: —H; C1-4alkyl; C2-4alkenyl; and halogenated C1-4alkyl; each of HX1 and HX2 is independently a protic acid; and pharmaceutically acceptable salts, solvates, and hydrates thereof. These compounds are useful as drugs, for example, in the treatment of tauopathies, such as Alzheimer's disease, and also as prodrugs for the corresponding oxidized thioninium drugs (for example, methythioninium chloride, MTC).