horse pill

noun2

1. •
   (humorous) A medicinal pill which is very large in size and is therefore difficult for a person to swallow.

   • I'm lucky enough to remember to take my horse pill, much less drop it into a glass and wait 45 minutes. — 2002
2. •
   A fact, proposal, claim, etc. that is difficult to accept or believe.

   • Even pro-Soviet Communists will find the Geneva accord a difficult horse pill to swallow. — 1967