Heroin earned the nickname “horse” because it kicks equines into overdrive. Morphine and other opiates, which lull humans to sleep, also trigger the ancient equine flight response.
In the wild, pursued by predators, a horse runs as fast as he can or dies. Given narcotics, a horse feels unnatural sleepiness creeping into his nervous system–sleepiness like the shock caused by the fatal bite of a carnivore. So the hopped up horse runs without reserve. If kept in his stall, he trots in circles until the dose finally ebbs. Let loose on a racetrack, he outruns any normal inhibition.
In the United States, cocaine, heroin and morphine were legal for anyone with a doctor’s prescription to buy from a drugstore, until prohibited by the Harrison Act of 1914, and could be bribed from pharmacists long after that. But using those mixtures was a fine art. Prudent trainers experimented during morning workouts, discovering the right dope and dose for each horse.
~ Dorothy Ours, in her book Man o’ War: A Legend Like Lightning