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
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
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 4, 2016
When you develop hatred for vaccines that you decide ivermectin is the way.
"I was afraid all of my friends were planning to take the Pfizer jabs. I told them the vaccines are dangerous, but they laughed at me and called me names. When they all took the jabs, I was really mad and lonely until someone gave me some dose of horse drug. That's when I became horse-pilled"
by Just_satire September 17, 2021
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