Narcissa looked at him with disapproval as she addressed Hermione's question. "The potion was designed to take your magical and physical reserves and to, in a word, concentrate it for a limited period of time. It can boost your strength, speed, and magical ability... at a cost. The duration of the potion as well as the cost in your overall life-span seems to be completely random and the benefits never outweigh the risk, Severus."