Brent's advice is good. I have hiked to the Whitney Summit 4 times. Only on one trip did I have issues. I had hallucinations and slurred speech, but did not know it until later (like Brent said ... compromises your judgement). No headache, no nausea, clear vision. It apparently started on the switchbacks. I am positive it resulted from my lack of eating. I was drinking fine, but not eating. If you can join in on the hike with others, that is a good thing. That interaction, as Brent pointed out, gives you feedback on how you are doing. I did that once during one my failures to reach the summit. It certainly helped me to decide to turn around.
