Propecia has been demonstrated to block the formation of dihydro-testosterone (DNT) in men with male pattern hair loss, and this can contribute to a decrease or arrest of baldness. Clinical studies have indicated that men who took Propecia maintained or increased the number of visible scalp hairs and many noticed an improvement in overall hair maintenance and growth. Side effects, although uncommon, include a loss of desire for sex and impotency in some men. Because Propecia can cause serious birth defects in the sex organs of unborn infants, pregnant women should avoid taking or handling this drug.