Quote from Wikipedia:
Sulbutiamine (brand name: Arcalion) is a synthetic derivative of thiamine (vitamin B1). As a dimer of two modified thiamine molecules, it is a lipophilic compound that crosses the blood–brain barrier more readily than thiamine and increases the levels of thiamine and thiamine phosphate esters in the brain. Sulbutiamine was discovered in Japan in an effort to develop more useful thiamine derivatives since it was hoped that increasing the lipophilicity of thiamine would result in better pharmacokinetic properties.
Although its clinical efficacy is uncertain, it is the only compound used to treat asthenia that is known to selectively target the areas that are involved in the condition. In addition to its use as a treatment for chronic fatigue, sulbutiamine may improve memory, reduce psycho-behavioural inhibition, and improveerectile dysfunction. At therapeutic dosages, it has few reported adverse effects. It is available for over-the-counter sale as a nutritional supplement.