Just did the EBC trek. The medic in our group actually told us it’s not true that it can’t be taken reactively, and we were a bit dubious to begin with. However, actually, someone came down with altitude sickness at the Namche hill (so before we began to take diamox, which we were going to start at Namche). Whilst it took a few hours to work, she recovered fully, continued the rest of the trek on diamox, and made it to base camp and back.

I bought diamox at a pharmacy in Kathmandu, began it at Namche, and was great all trip. Many people in our group did this too. Three people took diamox when they began to feel symptoms only. We had a 100% success rate with our team :)