I had a horse with navicular who was on daily bute, his dose was upped and about 3 days after he got ulcers in his mouth and diarrhoea due to a toxic reaction to the bute. We took him straight off it and they healed fairly quickly. It may be worth having a good look in his / her mouth to check there are no ulcers or anything.
Thanks guys. I have passed the info on to my friend. He has noticed a few small ulcers and his horse is a little uncomfortable in the mouth and it coincided with him having bute. Never known it to do this before.