Most often refers to very large pills. Big enough for a horse and hard to swallow.

However, in Thailand "ya ma" or "horse pills" usually refers to amphetamines.
The doctor prescribed me horse pills!
by Cottser July 14, 2006
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Classically, refers to size; an unusually large pill.

Realistically, should refer to Premarin, the birth control that consists of "conjugated estrogens" extracted from horse urine!
1. How on earth can you swallow that multivitamin; it's a horse pill!

2. She's on horse pills
by zaphraud October 26, 2004
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An abnormally large pill(s) or referring to the strength of the pill being a heavy dose. Ideal one would thing such a pill was mean for a horse instead of a human.
Cute nurse: Here is you medicine for today.
Old man: I believe you have my medicine mixed up, these are horse pills.
Cute nurse: If you do not want to take your medicine. I will grind them up and put them in peanut butter and stick it to the roof your mouth, is that what you want?
Old man: No, fine I will take them.
by Magnum_p.i. February 03, 2016
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A hockey puck, because of its shape and size.
Joanne got clocked by a horse pill at the Pirates' game last night.
by pentozali November 15, 2005
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