Precursors for thiamine, called sulbutiamine and benfotiamine, are very well absorbed and rapidly converted to thiamine after a patient takes a tablet. This offers the opportunity for a well absorbed medicine that can be taken each day by people who do not have enough thiamine in their body. This trial aims to determine where in the body sulbutiamine/benfotiamine is converted to thiamine. To do this, we are asking volunteers undergoing their routine TIPSS patency check to take a tablet just before their procedure. We would then take blood samples from the vein being checked during the procedure. This will enable us to measure sulbutiamine/benfotiamine and thiamine concentrations in the vein bringing blood from the gut to the liver and work out where the conversion happens.