Do you know how many beef cattle are injected with drugs from bottles that say "not to be used in animals intended for human consumption"? Most of them.
The thing is, unless you've worked in the cattle industry, you wouldn't know that ^^ that statement actually just means "don't shoot them up today and butcher them tomorrow". For most bovine medicines, there has been enough study to know what the withdrawal period is for the drugs they're being given. Most are 30 days, some are longer. That means that 30 days after injection, all traces of the drug are out of their system.
In spite of what many animal rights groups would have people believe, traces of drugs do not stay in an animal's body indefinitely. The reasoning for the label on the equine drugs is because withdrawal periods have not been set...because horses haven't been slaughtered for human consumption in the US in a long time.