Circe's potion is a magic drink used by the sorceress Circe that turns the drinker - most likely a male - into a guinea pig. It used to turn the drinker into a real pig, but they were too hard to handle, so Circe changed the potion's power. It can be countered by Hermes' multivitamins or Hermes' wine, as in the original myth.
This is a reference to the original myth, where Circe lulls Odysseus' crew into a stupor with food and drink, and then turns them into pigs.
Annabeth Chase and Percy Jackson arrive on Circe's Island after Clarisse's ship is wrecked. Annabeth is given special treatment, but Circe keeps Percy behind and tells him she can give him a makeover, and remove all his flaws. Percy is caught under her spell and drinks the potion she gives him, which looks and tastes like a usual strawberry milkshake, but actually is the potion which then turns him into a guinea pig. When Annabeth discovers what Circe has done, she uses Hermes' multivitamins to revert the spell, turning Percy, Blackbeard, and his pirates back into humans.