I took Diamox starting with the night I camped at Whitney portal and then the next day as well when I started my hike. 250mg every 12 hrs. I did not take it on my Summit day. Luckily I did not need a prescription as I had some extra supply from my trek to Everest Base Camp last year.
I did not experience any altitude sickness whatsoever which I solely attribute to the Diamox. The reason I say it is because I experienced terrible AMS when I climbed White Mountain Peak last year, at around 13,000 feet I almost became debilitated and could not walk without panting heavily.
For Mt. Whitney I was perfectly fine all the way to the peak! Diamox worked for me like magic. The only problem is that it makes you pee like crazy and you risk dehydration if you are not monitoring your fluid intake very carefully.
