Benfotiamine (S-benzyolthiamine-O-monophosphate) is a synthetic derivative of thiamin, belonging to the family of compounds known as "allithiamines." Benfotiamine is fat-soluble and thus more bioavailable and physiologically active than thiamin. Characteristic of the allithiamines is an open thiazole ring within the chemical structure of these thiamine-related compounds, making them fat (lipid) soluble. In contrast, thiamine, which is water soluble, has a closed thiazole ring. The lipid solubility of benfotiamine, conferred by this open ring, increases its bioavailablity. Benfotiamine is readily absorbed at higher doses, in contrast to absorption of water-soluble thiamin salts, which decreases at higher doses, due to saturation of absorption sites in the intestines. In a double-blind, cross-over trial, comparing bioavailability of benfotiamine to that of thiamine in 12 subjects, benfotiamine caused an average 5-fold greater increase in blood thiamine levels than thiamin mononitrate, with a concomitant greater thiamine concentration in erythrocytes (red blood cells). Benfotiamine readily passes through intestinal mucosal cells, where it is converted into physiologically active thiamine. Benfotiamine inceases blood levels of thiamine pyrophosphate (TPP), the primary thiamin co-enzyme.