Frequently in cattle. However, if given to a cow intramuscularly death follows extremely quickly (<10 mins, its deadly if it gets in a cow's bloodstream) - we put it in the cartiledge of their ear. Horses must be different, will have to look at the bottle tomorrow! Learn something new every day.
I like excede, however, find it obscenely overpriced. Excenel is the same antibiotic, but WAY cheaper. It needs to be given daily though. Excede is pretty much just slow release ceftiofur. In cattle, excede also has a 28 day beef withhold, while excenel (another ceftiofur, not sure if it can be used on equines though) is only 4 days. It works very well in our cattle because its a 'dose and forget' kind of drug. Keeps her from getting stressed during restraint and keeps us from having to poke her daily. I've used it with success for everything from foot rot, metritis, as a preventative to a lady that pulled most of the skin off the front of her hock, to pneumonia. It's a VERY strong 'wonder drug.'
I find it funny your vet would jump to such a potent anitbiotic right away. If you don't mind me asking, what is it for?