A lot of references for use of Diamox (Acetazolomide) in this thread, but I was surprised to learn that mountain guides on Rainier dispense it prophylactically en route on summit day, as everything that I have heard or read from high-altitude physiologists over the years is that one must begin taking the drug a couple of days before one reaches an altitude at which one really needs it.
I tried to use Diamox on Aconcagua, but found the tingling in the finger tips, toes, and cheeks a problem as it was masking sensations that I would associate with frostbite. So, I stopped taking at the advice of the basecamp doctor at Plaza de Mulas, took his advice to drink 5+ liters of water per day instead, and suffered just a mild head ache on the summit at 22,841 ft.
I think that the advice above on this thread to go slow (may be difficult if you are in a guided party that wants to go fast to summit early and avoid the slush on the way down), drink lots of water, and be in reasonable shape are more important than Diamox. As I noted earlier in this thread, being able to acclimatize a few days at altitude before attempting Rainier sure helps, but of course not always possible.
Here are a couple of quotes from a British website, but Wikipedia also has some nicely researched pages on Diamox:
“….seems to works [sic] by increasing the amount of alkali (bicarbonate) excreted in the urine, making the blood more acidic. Acidifying the blood drives the ventilation, which is the cornerstone of acclimatisation. For prevention, 125 to 250mg twice daily starting one or two days before and continuing for three days once the highest altitude is reached, is effective. Blood concentrations of acetazolamide peak between one to four hours after administration of the tablets.”
http://www.traveldoctor.co.uk/altitude.htm
http://en.wikipedia.org/wiki/Acetazolamide