half-life

Given the example:

“The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself, and you will have but a half-life, a cursed life, from the moment the blood touches your lips."
Harry stared at the back of Firenze's head, which was dappled silver in the moonlight.
"But who'd be that desperate?" he wondered aloud. "If you're going to be cursed forever, deaths better, isn't it?"
"It is," Firenze agreed, "unless all you need is to stay alive long enough to drink something else -- something that will bring you back to full strength and power -- something that will mean you can never die. Mr. Potter, do you know what is hidden in the school at this very moment?"
"The Sorcerer's Stone! Of course -- the Elixir of Life! But I don't understand who --"

* * *
Random House dictionary says ‘half-life’ is “a brief period during which something flourishes before dying out.” I think this is in conflict with ‘to stay alive long enough to drink something else’ in the example. Does this half-life have the long enough span of life?