It won't kill you, but it can make you sick. You have to drink water all day long. You want your pee to be clear. If it isn't drink more. I live at about 800 foot elevation, and go to coloardo every summer. I've climbed a few 14,000+ footers in the last few years. Expect to be out of breath (even if you're in great shape) and expect a headache for at least the first day, after that it should wear off.
-Jake