Probably the most common initial responses are signs of relaxation such as sleeping and yawning (as demonstrated by Fred here); but also snorting, sneezing, coughing and tickling in the nose. The horse may wish to rub its nose due to the tickling. The nose may drip or even stream. One nostril may be blocked early on.
With a regular 1N program at some point I expect the symptoms to diminish. How long this takes is very variable according to the individual.
If you do sufficient 1N you may also see healing responses such “Return of symptoms” which are well known in holistic healing. These returns are usually subtly different from the original symptom and don’t seem to bother the horse in the same way,
Another healing response, known as ‘clearing’ is thought to occur when the rate of healing is higher than the excretory systems can cope with. As in humans they include a runny nose, itchiness, diarrhoea, lack of appetite, increased thirst, or rashes. Clearings often pass quite quickly, sometimes in a few minutes and so may easily be missed but if they become a nuisance, or if your horse declines 1N, wait for a day or so until the body has caught up with the healing process, before resuming 1N. Consult your vet if in any doubt.