"Acetazolamide (Diamox) is the drug of choice for prophylaxis of AMS.... numerous dosage regimens have been effective Smaller doses (125-250 mg a day) started 24 hours before ascent work as well as higher doses started earlier. A 500 mg sustained action capsule of Diamox taken every 24 hors is probably equally effective and results in fewer side effects because of the lower peak serum level. Most authors recommend continuing for the first day or two at high altitude...."
source Wilderness Medicine by Paul Auerbach 4th edition.
I've never used. I have subcribed a few times and people seem to tolerate well. I have talked to hikers who have used without problems.