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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
74 3
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
90 27
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
8 61