- Diuretics refer to drugs that increase urine output. All diuretics function by increasing the amount of osmotically active solutes in the tubular fluid. Because water osmotically follows these solutes, urine outflow is increased. In most cases diuretics achieve this by reducing the tubular resorption of sodium although in some cases bicarbonate or the diuretic molecule itself acts as the osmotically active solute. Diuretics are sub-classified according to their mechanism of action which in most cases is focused on where along the tubule they exert their effect.