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1.
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
 
2.
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
 
3.
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