Introduction: Alzheimer's disease AD is a major cause of morbidity worldwide and its prevalence is expected to rise. Previous studies involving compounds that target the accumulation of amyloid protein have been unsuccessful, renewing interest in therapies directed against intracellular deposits of tau proteins. Derived from methylene blue, hydromethylthionine is a tau aggregation inhibitor that recently emerged as a promising disease modifying treatment for AD.Areas covered: Herein, the authors cover the chemistry, pharmacodynamics and pharmacokinetics of hydromethylthionine and its oxidized form methylthionine chloride MTC that was first studied, as well as clinical efficacy and safety of hydromethylthionine in the treatment of mild to moderate AD.Expert opinion: Randomized clinical trials with hydromethylthionine failed to show any impact of the doses used on the disease course. ...