You trade 8 Gold Bits for Antidote, and Firefly.

You put the firefly in your Old Lantern.

Shopkeeper: "Anything else?"

You ask the shopkeeper about mare's poison.

Shopkeeper: "You're not from around here, are ya? Heh, well if you gotta ask, it's a medicine based on poison joke. Flushes your heat cycle right outta your system!

Don't worry, I'm makin' it sound worse than it actually is. Matter of fact, it's pretty popular, and not just with the loners. Allows a mare to spend a casual day with a stallion friend. Er, should I say night? Anyway, you get the gist. No foal, no foul."