I read this on Slate in an article about Barbaro's recent broken leg:
It's not easy to treat a horse with antibiotics, either. Since the animals are so big, you have to pump in lots of drugs to get the necessary effect. But if you use too many antibiotics, you'll destroy the natural flora of its intestinal tract, which can lead to life-threatening, infectious diarrhea. You also have to worry about how the antibiotics will interact with large doses of painkillers, which can themselves cause ulcers.
So, they worry about giving horses digestive tract problems from antibiotics, yet doctors give out antibiotics with impunity to people and then scratch their heads and shrug their shoulders when we get digestive problems after a round of antibiotics?
Just one more reason to talk to your vet about your medical problems, LOL.