I have been guiding the Himalaya for going on 20 years now. In 20 years we have prescribed Diamox twice and we have had only 1 person not acclimatize mainly because she had food poisoning from chicken and could not keep her strength up.
If you are going to altitude the key to acclimatization is in the rate of ascent. If you join a company for a trek or expedition in the Himalaya and they prescribe Diamox for the duration of the trip you need to question there rate of ascent, itinerary of that company, and experience at altitude. I want to be clear here. I am speaking about trips in Nepal where you can control your rate of ascent and starting points are below 9000 feet. In this environment you do not need Diamox.
A few things to keep in mind.
1. Everyone will acclimatize if all the rules of altitude are followed. If you would like information on these rules feel free to email me.
2. Your rate of ascent per day should be less than 1,400 vertical feet. On days where you move higher than this follow it with a rest day. On rest days do not lay around, do a day hike gaining 1000 feet or more and at your high point try to hang out for an hour or more.
3. Sleeping propped up helps with circulation of fluids and oxygen exchange.
4. When you arrive at your new altitude the first inclination is to lay down. This is fine for 10 to 20 minutes to get settled but then you want to walk around, move, and be upright for most of the day.
5. Drink at least 2 quarts of water per day. This is the most important rule. Tea, coffee, and soda drinks do not count in your water intake.
6. Keep your caloric intake up even if you are not hungry. We take a brake on the trail every hour to nibble on an energy bar or what have you, and sip water.
These are just a few rules but these are the main ones that will get you acclimatized. Diamox should only be used if someone is not acclimatizing which is rarely if the rules are followed. I keep Diamox in our high altitude kit for back up. Think about it this way. If you are on Diamox and you still are not acclimatizing you have nothing to fall back on, trip over. If for some rare reason you are not acclimatizing you can take a rest day, get on Diamox and possibly proceed within the next day or so. It is a good drug to have for back up but it is not needed if you follow the rules.
I nor any of my colleagues, friends who guide Everest, K2, and are high altitude pros never prescribe Diamox from day one on any trip in the Nepal Himalaya. If a doctor or company suggest such a regimen you may want to question their itinerary, rate of ascent, and years of experience at altitude. Often in the Doctors case they have read books and studies but have not actually been to altitude many times. Prescribing Diamox from day one is like prescribing a bandage for a cut that has not happened. If the cut does happen then you have no bandage to use because it has already been worn.
Keep the Diamox in your altitude kit as back up. Remember you will acclimatize without it if all the rules are followed.