This is what a vet told me years ago about using banamine. When a horse feels pain/ discomfort - his digestive system slows down. So now that his digestive system has slowed down he is having pressure from gas and the contents of the intestines that are not moving and this pressure causes him discomfort. That pain/ discomfort slows his digestive system further. Sometimes. if banamine is given early enough in this viscous cycle the horse is not so painful and his digestive system starts working again like it should. This makes sense to me. Of course if you wait until the horse has been painful for a long time this won’t work. Also banamine is not going to fix a torsion or twist. However these can happen because of the backed up gas and ingestia in the intestines. 5cc of banamine will not mask that much pain.
The vet clinic I use now urges their clients to use a “colic pak” when your horse becomes painful. Of course after you have talked to one of the vets and discussed symptoms with him. If that doesn’t work pretty fast they recommend a trip to the clinic where the horse can be monitored round the clock and given fluids as needed. As a general rule the faster you notice something is “off” the better the outcome. They are not proponents of endless walking. And they see a LOT of colic cases.
However sometimes there are internal things going on that the horse can’t tell you about. So no matter how you treat the horse it is not going to make it. This sounds like one of those times.