If you have a history of AMS, it probably pays to take Diamox before you go up. Otherwise, carry it and take 125 mg if and when you need it.
I am not a doctor but here's my dosage advice based on having taken diamox multiple times and having read a ton of literature...
125mg per 24 hours is the optimal preventative (prophylactic) dose. As mentioned, splitting tablets is fine. You MUST start 2-3 days before being at elevation for it to make a difference. At this dosage side effects shouldn't be more than increased urination and possibly some light tingling in extremities. (Assuming, of course, that your doctor has cross-checked vs any other conditions or medications you may have which might interact w/ diamox.) 250mg/day will give you much more pronounced side-effects with minimal to no additional preventative effect.
Since you already have the prescription you may as well take it preventatively rather than wait for any symptoms.
Taking 125mg after you are already experiencing symptoms won't do enough. At that point you need 250 or 500 / day and this means you are already in an emergency situation and side effects are the least of your worries. You need to get down from altitude ASAP and possibly get to a doctor.
I have found that doctors not familiar with mountaineering routinely get confused between the recommended dose for prevention vs. acute treatment. I had one doctor give me 500mg/day prescription for prevention which would have been WAY too much.
I don’t know where you got your Diamox knowledge, but I got mine from two professionally guided trips on Denali. Mountain Trip had us bring Diamox in 1994. The norm was to wait for symptoms to appear and then start taking ¼ to ½ of a 250mg tablet twice a day. We all eventually took Diamox, and symptoms pretty well disappeared once we took it. We turned around at 18,360’ due to marginal weather and three climbers performing poorly, not due to AMS.
The Diamox story was similar in 1995 with American Alpine Institute. Lead guide
Steve House took a lot of time to explain acclimatization, taking care of ourselves at altitude, and, yes, Diamox and other drugs. As in 1994, everybody eventually took Diamox, and symptoms subsided. I was proud that I was again the last team member to need it. I had Cheyne-Stokes breathing one night. I took ½ tablet and went right back to sleep. Problem solved. I took ¼ tablet every 12 hours after that. I slept fine at 14,700’ and at 16,100’ the next few nights. We had good weather for summit day, and every member of our team reached the summit.
Fast forward a decade or so, and somebody figured out that taking Diamox prophylactically was beneficial. A quick internet search today revealed recommendations for taking Diamox one or maybe two (not three) days before ascending. This is probably a good idea for people with a history of AMS. But I frown on the idea of popping a bunch of pills because someone won’t take the time to acclimatize.
I have always recommended trying to climb drug-free. And I don’t do this because I lack knowledge that you possess. Quite the opposite.
