Beer1, the prescription will likely be for 1 250 mg tablet twice a day, beginning several days before ascending to elevation and continuing several days after descending. At least that's what it was for my wife and daughter for their first attempt to hike above 12,000 feet this year. Most people on this board will tell you to moderate that dosage and schedule, and I agree.
The tabs are scored in quarters, so you can chop them down to 125 mg or 62.5 mg. The dosage needed can vary by individual, but the main thing to be aware of is that the noticeable side effects (frequent urination, tingling in the extremeties, and taste sensitivity) increase with the dosage. Most folks I know who use Diamox in the mountains take either 62.5 or 125 mg twice a day beginning the day before ascent above 11-12K', and stopping the regimen once below the target elevation.
I used the stuff myself for the first time just last month. Elevation has never affected me as long as I acclimate at least 2 nights at/above 8000 feet. This time, though, with my wife and daughter attempting elevations they had never hiked to before, I elected to go on Diamox with them - just to be sure I had the best available capacity if either of them got in trouble in some way.
We all took 125 mg the night before acsending above 12,000 feet, and for the duration that we were above that elevation. We did this for Mt. Dana (13K') as well as Mt. Whitney. None of us experienced any AMS symptoms, nor did we feel any of the bothersome side effects of the drug. I should note, though, that we spent 1 night at 8000 feet just before Dana, and a total of 5 nights above 8000 feet immediately before Whitney. Diamox is a great aid, but it ain't a magic bullet - be sure not to bypass acclimation. We ran into several folks on Whitney who were retreating with AMS who were on Diamox. If you combine Diamox with proper acclimation, you've covered your bets as best you can.
Still, be sure to discuss with your physician - your mileage may vary.
