My horse has been taking the apple flavored stuff prophylactically since May before long cross-country trips in the horse trailer or before visiting with larger groups of horse-in-laws. He did have some powerful diarrhea the first time, but not on subsequent treatments. He often avoids other horses anyway, so it's hard to know how effective it's been for horse viruses.

However: This horse had some really bad knees and ankles a couple of years ago, to the point where it was challenging to climb stairs or walk very quickly at all, and while that had improved a little over time, it's almost completely gone away over the summer and the horse is thinking about starting running again. Neigh!