A methylthioninium ("MT")-containing compound for use in a method of therapeutic or prophylactic treatment of a neurodegenerative disorder of protein aggregation in a subject, which method comprises orally administering once per day to said subject said MT-containing compound, wherein said administration provides a total daily dose of between 0.5 and 20mg of MT to the subject per day, wherein the compound is a compound of the following formula ("LMTX"): wherein each of H n A and H n B (where present) are protic acids which may be the same or different, wherein p = 1 or 2; q = 0 or 1; n = 1 or 2; (p + q) × n = 2, and wherein said neurodegenerative disorder is selected from the list consisting of: Alzheimer's disease; Pick's disease, progressive supranuclear palsy, frontotemporal dementia, FTD with parkinsonism linked to chromosome 17, frontotemporal lobar degeneration syndromes; disinhibition-dementia-parkinsonism-amyotrophy complex, pallido-ponto-nigral degeneration, Guam-ALS syndrome, pallido nigro luysian degeneration, cortico-basal degeneration, dementia with argyrophilic grains, dementia pugilistica or chronic traumatic encephalopathy, Down's syndrome, subacute sclerosing panencephalitis, Niemann-Pick disease type C, Sanfilippo syndrome type B, or a myotonic dystrophy DM1 or DM2; Huntington's disease, spinal bulbar muscular atrophy, dentatorubropallidoluysian atrophy or spinocerebellar ataxias; A TDP-43 proteinopathy which is FTLD-TDP; Parkinson's disease, dementia with Lewy bodies and multiple system atrophy; Hereditary cerebral angiopathy; Amyotrophic lateral sclerosis; and familial encephalopathy with neuronal inclusion bodies. which method comprises orally administering once per day to said subject said MT-containing compound, wherein said administration provides a total daily dose of between 0.5 and 20mg of MT to the subject per day, wherein the compound is a compound of the following formula ("LMTX"): wherein each of H n A and H n B (where present) are protic acids which may be the same or different, wherein p = 1 or 2; q = 0 or 1; n = 1 or 2; (p + q) × n = 2, and wherein said neurodegenerative disorder is selected from the list consisting of: Alzheimer's disease; Pick's disease, progressive supranuclear palsy, frontotemporal dementia, FTD with parkinsonism linked to chromosome 17, frontotemporal lobar degeneration syndromes; disinhibition-dementia-parkinsonism-amyotrophy complex, pallido-ponto-nigral degeneration, Guam-ALS syndrome, pallido nigro luysian degeneration, cortico-basal degeneration, dementia with argyrophilic grains, dementia pugilistica or chronic traumatic encephalopathy, Down's syndrome, subacute sclerosing panencephalitis, Niemann-Pick disease type C, Sanfilippo syndrome type B, or a myotonic dystrophy DM1 or DM2; Huntington's disease, spinal bulbar muscular atrophy, dentatorubropallidoluysian atrophy or spinocerebellar ataxias; A TDP-43 proteinopathy which is FTLD-TDP; Parkinson's disease, dementia with Lewy bodies and multiple system atrophy; Hereditary cerebral angiopathy; Amyotrophic lateral sclerosis; and familial encephalopathy with neuronal inclusion bodies. Alzheimer's disease; Pick's disease, progressive supranuclear palsy, frontotemporal dementia, FTD with parkinsonism linked to chromosome 17, frontotemporal lobar degeneration syndromes; disinhibition-dementia-parkinsonism-amyotrophy complex, pallido-ponto-nigral degeneration, Guam-ALS syndrome, pallido nigro luysian degeneration, cortico-basal degeneration, dementia with argyrophilic grains, dementia pugilistica or chronic traumatic encephalopathy, Down's syndrome, subacute sclerosing panencephalitis, Niemann-Pick disease type C, Sanfilippo syndrome type B, or a myotonic dystrophy DM1 or DM2; Huntington's disease, spinal bulbar muscular atrophy, dentatorubropallidoluysian atrophy or spinocerebellar ataxias; A TDP-43 proteinopathy which is FTLD-TDP; Parkinson's disease, dementia with Lewy bodies and multiple system atrophy; Hereditary cerebral angiopathy; Amyotrophic lateral sclerosis; and familial encephalopathy with neuronal inclusion bodies.