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.
Source: Dorothy Ours, in her book Man o’ War: A Legend Like Lightning »

Last Updated: 12 May 2020 — Featured Image: Grey horse in poppy field.
my theory is that, when you fall off a horse, the only way is to get back on the horse, thus back on heroin, a bit like “going on and falling off the waggon” with alcohol. yo yo dependance.
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[…] recap: Horseface, drugs. Type “horse drugs” in Google, first page, you get this article and learn that another name for heroin is horse. Bingo! Now you know what the Maestro really means […]
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[…] No, don’t worry, that’s not code for a heroin habit. Interesting story behind that whole bit of slang, though. […]
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[…] by ‘Lethal White’ is almost certainly it. [For those of you not savvy to street idiom, ‘horse’ is vernacular for ‘heroin,’ just as, curiously, is the name ‘Harry.’ I kid you […]
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I did NOT know this. Thanks for posting. I guess there is nothing some people won’t do to win….
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