In the US at least, horses are not raised for meat. This means that veterinarians use whatever drugs they deem necessary when treating horses many of which might make the meat toxic.
I've had this told to me by several horse people including my wife and a horse vet but don't know how true it actually is.
The dangers of drugs in horse meat are vastly exaggerated by those opposed to eating horse meat. The thing to remember is that drugs, especially those that might pose a problem, are expensive. As such, they might be in common use for e.g. valuable Thoroughbred stallions in Lexington KY but are vanishingly rare for swaybacked old "killer" horses fending for themselves on weeds and thornbushes on someone's back forty. Sure they might have been "worth" such drugs when they were younger, but that was years ago. Experience from cattle drugs indicates that everything is out of the system in 60 days.
I've had this told to me by several horse people including my wife and a horse vet but don't know how true it actually is.