Prosultiamine (Alinamin®), a well-known thiamine derivative, was first developed by Takeda Pharmaceutical Company in Japan in the 1950s. The drug is a homolog of allithiamine produced by thiol-type vitamin B1 and allicin. Prosultiamine is converted to vitamin B1 after absorption from the gut. The drug thus enables a long-lasting high blood concentration of vitamin B1, resulting in efficient access of vitamin B1 to nervous tissue. Prosultiamine has cured many patients with vitamin B1 deficiency resulting in beriberi neuropathy and Wernicke’s encephalopathy. Prosultiamine is also a potential treatment for HTLV, since it has been shown to reduce viral load and symptoms.